Tutorials/Mob farm

The purpose of a Mob Farm is to automate and maximize the process of spawning mobs, killing them, and funneling the resulting items to a collection point. There are many designs, each with advantages and disadvantages.

Canal-style Farm
This farm will work in 1.2 ONLY if you place open trap-doors or signs on the edges of the canals. The mobs will fall through, because they think the open trapdoors/signs are solid blocks.

This type of farm is a large underground cavern with interlocking canals used to funnel mobs to a central hole. From there, the mobs are transported to a simple lava killing device, and the resulting items are filtered downstream to a collection point. This collection point is far enough away from the cave that you can AFK there without disrupting mob spawns.

Inspiration for this tutorial, and original concept, was made by Mokomi: (http://www.youtube.com/watch?v=HdKBeInv_2s).

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Digging the Shaft
Your first task is to dig downward, a straight shaft is recommended. You will want to dig all the way down to bedrock. Hollow out a small room of about 5x5x3 blocks. This will be your target for the collection of the items.

Now count blocks back up your initial shaft. You want to dig another room with the floor being the 16th block up from bedrock. If you want to be safe, err on the side of going higher (17th block is fine). You want to make sure you have enough room to not only dig your canals, but also the resulting funnel and item collection stream. Remember, your target is that little room on bedrock!

Make another small room, about the same size, right above the other room on bedrock. This will be your base of operations, so get comfy! Place your workbench, furnaces, and chests here in whatever fashion pleases your design sense. Then, dig a 4-block-long hallway into the wall adjacent to the mine shaft. This will allow you room to place a door, but also enough room to fill it back up with dirt or stone later. You will want to remove all light sources into the farm when you are done.

Digging the Cavern
Now comes the tedious part! You will need to dig a 50x50x2 cavern. Everything must go! I kept my little hallway in the corner of the 50x50 plot. Then, just pick a direction and start digging out. If you choose to dig out and right, your end funnel will need to come back left to meet up with the basement collection point, and vice-versa.

It is also recommended that you keep the height to 2 blocks to promote more sideways movement of the mobs, and prevent them from jumping over or out of your canals. If you hit gravel, fill in the hole to make the ceiling level.

If you want, you can alternate digging the flat cavern with digging the canals. If you choose to do this, read through the rest of this guide to understand the whole project first.

And most importantly KEEP THE CAVERN WELL LIT!! You will be working around in a large underground area. You will, at some point, want to return to the surface, leaving your cavern to become prematurely populated with mobs, and that's a bad thing. I put torches on the wall every 5 blocks (also using double torches to help mark canal placement) and in the central squares, I generally used a layout much like the '5' on a six-sided dice. It might also be a good idea to keep the difficulty on "Peaceful", so you don't have to worry about it. Having the difficulty on Peaceful will also not deplete your hunger bar, so you do not have to worry about food while digging.

Alternatively, you can dig access tunnels underneath the platforms and remove the blocks holding the torches from underneath and put down new blocks in their place. Such areas can also be useful for storing chests.

1.8

However if you want Endermen make the cavern 50x50x3 because they are 3 blocks high. Endermen will take damage on contact with water. NOTE that Endermen may grief your cavern, so take note to fix it periodically. Endermen are able to pick up blocks, and upon death the blocks disappear. However, they can not pick up snow blocks or double slabs of any kind, so the walls (including canal walls) should be made of these materials, two blocks thick, so that they can not pick up any blocks.

1.9 prerelease 2 or further

Endermen are no longer able to pick up stone or cobblestone, so make your walls (including canal walls) out of those materials, two blocks thick.

Important note: Since 1.9 prerelease 2 Endermen will immediately teleport upon coming in contact with water. So if you are planning to use canals to transport mobs, then making the roof 3 blocks high is unnecessary.

Digging the Canals
Great, depending on how you decide to tackle this project, you may be finished with the whole cavern by now. If so, awesome! Now is time to start digging our canals. They will cover a good area of the floor and use water streams to push mobs into the center. I have included a map of sorts for digging these canals, as the height and placement of the canals is important for keeping the water flow constant and free of any "dead zones", or still water, that mobs can get stuck in.

There are a couple of keys to digging your canals. First, always precede any intersection with at least two blocks that are level with the intersection. Having water *fall* into the intersection causes some weird water flow and can cause those dreaded "dead zones".

Second, the ends of the main canals, or the ones that form the large cross in the middle, start 2 blocks below the floor, not one. This is, again, important to maintaining a good flow throughout the canals.

You may feel tempted to test out your canals before the project is complete. While I normally recommend testing your project as you go, I might try to resist the temptation here. By having water flowing through your canals as you are constructing them, you could find yourself fighting against the current, severely slowing your work. If you must test, make sure to remove the water before continuing your dig.

Adding the Trapdoors/Signs
In order to have mobs walk off the solid ground into your canals, the edges of the canals need to be lined with "open" blocks like trapdoors or signs. Trapdoors are approximately twice as resource efficient as signs (6 planks versus 12 planks and 2 sticks for 2 items of each), which depending on the size of your canals can equate to a large savings. Trapdoors are a bit trickier to place and set up, however, once placed you can easily navigate throughout your canal room while building. Essentially, anywhere you would like a mob to walk off into a canal, you must place a trapdoor/sign on the wall of the canal. Do this for the entire length of your canals for maximum effectiveness.

If using trapdoors, make sure they are all in the "open" position.

Door based mob funnel
An alternative to digging canals to funnel mobs to a cental shaft is to place doors with pressure plates next to them so that the doors are open but shut when mobs step on the pressure plates, essentially making a one-way door. Make a wall of these in several concentric squares. Then, as mobs spawn, they will eventually wander inward through the series of door walls. As they walk away from the shaft, they are stopped by doors. Space these walls 2 or 3 blocks apart for best results. Eventually the mobs work their way to the central shaft. The disadvantage of this method are that it takes more resources than a canal trap, and mobs are moved to the funnel hole more slowly. The advantages are that this trap can be used on the surface to capture naturally spawned mobs, and it can be used in conjunction with canals to make a very effective ground floor for a mob farm. Additionally, this method works on Endermen.

Note: Use wooden doors, not iron doors, to avoid trapping yourself.However, if you are playing on Hard, zombies will break the wooden doors, so either turn down the difficulty or use iron doors.

Digging the Funnel
We're almost ready to flood these canals, but first we have one more project: the funnel. This is a tunnel that will start at the hole in the middle of our farm and extend out. Remember, if you started digging out and right from your initial base, this funnel will need to head back left, and vice-versa.

Start by digging down 3 blocks in the center 2x2 area of the farm. Then begin digging your 2 tall 2 wide hallway. You will be digging over by 7 blocks, then dropping down one. This will keep flowing the water from just two source blocks at the top of the funnel.

Dig over 7, down 1 three times. At this point, dig over 8 this time, and down another one. This last 8 will prevent water from bumping into the ladders you will be placing later. At this point, you will want to create your lava trap. For that, see the above image for the funnel cross-section.

You will want to dig a 2 wide, 1 tall, 3 deep hole that is level with the ceiling. Place two ladders below this hole (signs work just as well). This will suspend the lava in the air, but allow mobs to flow right into it.

Once you have your lava killer ready, it's time to start digging the collection stream. This stream will only need to be 1 wide and two tall (only so you can fit through to dig it!). You'll place a water source block under the lava on the opposite side as your collection stream. Remember, you're trying to aim back towards your original bedrock room!

You should have to dig about four sets of 7 over, 1 down to reach your room. Once there, you can do almost anything from just letting the water spill out the items, to placing a system of pressure plates and redstone wire to alert you to the presence of items. This might be a bit overkill, as I can assure you if everything went according to plan, you will ALWAYS have items waiting for you!

If no drops are coming out after a few minutes, try swimming up your tunnel and see if mobs are getting stuck on the ladders. if this happens try changing the ladders to signs, and if you want write a taunting message to the creepers that are dying in vain.

Here is another set up of a lava blade.

For more information on killing mobs at a central location, see mob grinders

1.8

You do not need to alter the funnel for the Endermen in 1.8; because they take damage on contact with water and the funnel is 2 blocks high they will get stuck in the water and eventually die.

1.8

Due to the possible inclusion of experience for mob killing by the player (your grinder will not provide this) It may be desirable to create a safe(er) environment in which to kill the mobs. (this method is for use with the bow)

To do this, place a sticky piston, powered by a lever, by your lava blade, so that it may be turned on or off.

Line the floor and walls of the last section of your canal with obsidian, to prevent any accidental creeper explosions from damaging your trap. Now, set a 1x1x3 hole in the side of the obsidian, looking onto a place where mobs will become caught in the stream (this is usually the spot right by the collection stream) This will be your "killing hole". Also, leave a hole in the top of the obsidian, above the area they become stuck, for light, and to clear the area should the player run out of ammo (unlikely). It is recommended this is also 1x1x3 so that it is harder for skellies to hit (though the player will be much more vulnerable here than at the kill hole).

From the kill hole, you should be able to clearly see and attack into the funnel, though it is still possible for skeletons to see through and attack. It is very unlikely to be hit, though it is still possible.

Ammunition should be plentiful, and if the player starts to run short, turn on the lava blade again, and drop a TNT into the hole to clear any leftover mobs. Collect arrows from the usual flow of items, then carry on killing! This should provide a safe and easy way for you to kill mobs, without placing the player in too much risk.

(NB: This also allows for the player to kill any stray spiders that make it through the trap)

Placing the Lava
Now it's time to light the fires! This is best done before filling the canals so you aren't fighting a current when placing these blocks. Place two lava source blocks in the back of your lava killer. If all went well, you should have a thin layer of lava suspended above the ladders you placed. Once that's done, head back upstairs for the final steps.

Filling the Canals
Now it's time to get this farm working! Remember those two buckets of water you came down with? Time to put them to use! Create an infinite spring somewhere in your floor. Near the middle might be a good idea! Then drop water source blocks at the very ends of all the canals. If you have extra iron at this point, make up a few extra buckets to speed up the process.

Once all the canals are started (this is a total of 32 water source blocks) take a walk around your farm and check for "dead zones". If you see any standing water, you're in trouble. Remove water source blocks to drain the affected area and check the depth map again for any irregularities.

If you find that the water freezes, you can place some water 1 block higher, allow it to freeze, then place some more water below the ice block. Running water will not freeze and water below ice will not freeze.

Removing the Torches
Now it's time to plunge your Mob Farm into darkness. Remove all the torches in the cavern in a circle around. Start at your base hallway, pick a direction, and run! Inevitably mobs will spawn during the process, but hopefully your canals will funnel them safely away. You'll probably have items waiting for you by the time you are finished! If you want to be safe, just pack all your tools and such into your chests so it's not terrible if you die.Or you could also switch the difficulty to peaceful while you build!

Covering the Door
The final thing to do is to place a couple of blocks (your choice) over the entrance to the hallway to the farm. To maximize mob spawns, you don't want any light getting in, even through those tiny door windows. I also place a sign on this section reminding myself not to dig there. This can also come in handy on a multiplayer server to prevent others from stumbling into your farm (as hilarious as that might be!)

Final Thoughts
This project demands a lot of work! It took me about 2-6 total hours of work, but I was trying to figure everything out as I went. You might find it takes you significantly less time with this guide.

You can, if you want, create some rooms looking sort of like bathrooms. With the length of the funnel, placing a couple torches down here in a glass room won't affect the mob spawning too much and it will give you a front row seat to watching the mobs burn!

I also hooked up a redstone repeater and wire system leading back to the surface (This can be accomplished by using a lever hooked up to a vertical transmission redstone circuit leading to a (labelled) redstone torch on the surface or in your base). Since the mob farm is constantly kicking out items, this light should always be on. It's more helpful in showing me if the farm ISN'T working properly, or when the light is off. This can tell you if something is wrong with your farm and that you should check it out.

Make it your own and if you make any significant changes for the better, feel free to let me know on the Talk page! Happy farming!

Optional xp, or Auto Grinder
It is possible in 1.2 to use the canal system to create a mob trap that will both grind and allow you to kill mobs for xp.

First create a "conveyor belt" to do so simply place ( in this case a large amount) of redstone repeaters in a square or rectangular fashion. Hook up all the corners with a piece of redstone. Then nearby build a 2X2 shaft extending 24 blocks high into the air. on top of the shaft put in your canal, or double canal system, and part way through place 4 ladders around the shaft and water on top so the mobs don't die when impacting the ground. then place 4 regular pistons in a 2X2 square on the side nearest the repeaters on the shaft, constituting one of the walls. to the left in line with the block just in front of the 4 pistons place 2 sticky pistons in a 1X2 pattern and then 2 regular pistons sitting directly in front of them. Facing the same side as the pistons last placed, go to the left and in line with the block in front of those pistons place another 4X4 square of pistons, and to the left another 2X1 and 2X1. so that you'll have a rectangle of pistons, with a line making a rectangle of blocks inside them. then fill in the blocks all the blocks in a 2X1 rectangle following the imaginary line and leaving the block directly in front of three sets of pistons un-blocked.

Now, you want to connect the corners of the earlier set of repeaters with the matching corners of the piston rectangle so the blocks travel in a rectangle. Hopefully you made the rectangle large to give your self a gap for the option of killing mobs. naturally two blocks closest the 2X2 set of pistons on the side of the shaft will be open half of the time, giving mobs a space to fall into. here dig one block out right next to the shaft and the bottom block of the shaft wall just above it so you can see into the gap where the mobs feet will be. in the time that the conveyor takes to fill the gap you can kill the mob for xp, but when it does fill the gap it will suffocate the mob killing it as well.

Notes on Optional xp Grinder
-It is not spider friendly, also creepers may destroy parts of the machinery if you don't kill them quickly enough

- Endermen can take blocks out of the side if not built with the proper material

- This is a complicated and expensive build, so unless you are very established in your world, and familiar with redstone, I would suggest not trying to accomplish it.

Multi-Level Canal
This design is useful if you are low on space; although it is also useful for building in a specific area or getting varied products (see below). Basically, you take the above canal-farm and make multiple, similarly-sized layers in a smaller floor area for each, instead of 50x50.

If you simply want to create more drops, you could add on 1 or 2 extra layers, with the 50x50 spawn areas 7 blocks above the spawn room underneath, with the central collection hole in the middle dug out so it is in the ceiling of the below spawn layer.

Building
There are multiple tricks to getting the most out of the multi-level canal farm. First, start building the first floor somewhat near the surface. 30x30 is a solid floor size to start with. Next, you should think about how you are going to make the rest of the farm. You can make it getting smaller or smaller as you go down, or you could make only two floors, but larger ones; it's up to you. To get the most out of your farm with multiple layers, you have many options to choose from. A good way to arrange or design the different levels is to vary them, meaning that you can have the aggressive mobs (zombies, skeletons, creepers, and spiders) on top, and then have the passive mobs (cows, pigs, chickens, sheep, but not squid; this will not work for squid very well, unless you want to make a whole floor water which would probably just be a waste of time since you will only be able to have one mob for that floor, reducing efficiency) underneath, giving a plethora of different items from your farm. In addition, you may also want to combine this farm with a sugar cane or cactus farm...or both!...to completely maximize your production levels for larger amounts of resources. After all the planning start making the trenches like in the example above. For multiple levels, however, you will need to be creative to keep the mobs in the tube going down while also letting the other mobs in to go down with the others. My suggestion is to actually build the chute in the corner of the floor, rather than dead center because it needs to go through multiple floors. This will let other mobs in while letting the ones coming down already to keep going. To make things run even smoother, you can make two shafts/chutes. This lessens the chance of a jam-up even more. Another helpful/useful thing to make is a door connected to a lever to the entrance of your "lava killer". This is always good to have if you want to close your farm for a little bit or just want to let them pile up and then reap that big pile o' stuff that floats down the chute.

Compact Mob Trap
This specific mob trap was created by MinecraftFan1 (Roofless version can be seen here: imgur. com/2GBGd, put it in your address bar and remove the spaces). This design is an incredible 20x20, yet can provide up to 2500 items an hour if you use a flat grass world (total may vary in regular worlds, depending on how you modify the world itself). This design is based on an above ground structure, not a below ground structure.

1. Start by climbing up high into the air. Preferably about 24 blocks into the air so you can wait below while mobs spawn in the trap.

2. Build a 20x20 square floor.

3. Build a five block high wall around the edge of the square. This should make the inside area 18x18.

4. In each corner, build an 8x8x2 raised floor. If done correctly, you will have four two block wide canals that meet in the middle.

5. Dig out the 2x2 intersection of the canals to create a hole for the mobs to fall through.

6. At the end of the canal, next to the wall, place water to create a water current which flows towards the plughole. If correctly done, the water will not flow out of the trap, but will stop on the edge of the plughole.

7. Construct a one block thick ceiling in line with the top of the wall. This will restrict the spawning floor height to two blocks.

There you have it! A basic monster trap! For an even larger amount of mobs, repeat steps 3-7 again and again. Since the ceiling is already constructed, you don't need to build another 20x20 square floor again.

A four level mob trap like this accumulated nearly 13000 items an hour if you have an entirely flat grass world (tested under the following conditions: Play on a flat grass world on peaceful until sunrise, turn difficulty onto hard, spend 10 minutes under the trap, collect all the items, then multiply total number of items by 6. Approximate total will vary)

Word of warning: Since you use water, the occasional squid may spawn.

You can also combine this with the quarry style of mining. If you dig a 19x19 shaft all the way to bedrock, you can stack many layers on top of each other, and put your grinder on the bottom layer. The stone from the quarry itself will be more than sufficient building material, and the farm itself is located underground, making it much less obvious on the surrounding landscape. The only disadvantage is that the items are deposited all the way down on the bedrock layer.

(to make this into an experience farm, don't construct so high, or build a raised platform for the mobs to fall on. Falling will damage them enough that one blow will finish them off and you will be able to collect the experience)

Small Vertical farm
Introduction

A small vertical farm, as the name suggests, is 17 blocks wide (including walls), is not spider-friendly, and can be made as long as you want it to be.

Building

First, set up the square with each side being 17 long. Once you have done this measure 8 blocks away from the side where you will place the water, and dig a trench at least 2 blocks down.

Once you have made the trench and canal system, fill the sides of the square that will not flood in the trench and connect it to a grinder system.

Then build up the base of the square until it is 4 blocks high. When there are 2 blocks in the layer below, lay strips of material length and width wise so that there is 1 block space separating them.

The result will be a single block hole with 8 squares next to it (sides and points). Every layer you go up alternates so that they don't match the layer below them; otherwise mobs might not fall down or could get killed.

Once you have done this you should put a roof on top and wait for the mobs to start dying.


 * note, it is better if it is short, but very long because the layer could get clogged up with mobs

Reason

The main reason to build this farm is if you are needing a supply of mob materials without hunting, but cant be bothered or you are in the wrong place (I.E, a base used only for travel) where making a full sized farm would be unnecessary.

Super Mob Farm
A super mob farm is a farm that is capable of creating almost every mob drop in the game (except for slimeballs and Nether mob drops)

Preparation
The basics start off with building the standard 50x50 hostile mob spawn room and grinder. You will need to consider whether you want to upgrade it to a dual spawn room by adding another chamber above (as described in the Multilevel Canal section) or not. This is crucial, as you will not be able to upgrade the hostile mob portion of the trap after construction once the next stage has begun. Adding the layer will require over double the amount of time for the preparation stage, but it will also double the hostile mob drop output, so choose wisely, depending on your needs.

2nd Stage - Passive Mob Trap
UPDATE: this may not work in versions of Minecraft above 1.8 because animals now spawning once, during chunk generation. Animals only spawn after chunk generation on very, very rare occasions.

To start, build a similar 50x50 spawn room with the canals in place, and the central pit dug out so it hits the layer below it (as described in the Multilayer Mob Trap section). Then, dig out one layer of the floor on only the flat surfaces, excluding canals. Replace this layer with a layer of dirt. Line the cavern walls with torches on exactly every five blocks. Continue this pattern on the dirt floor too, making a huge grid of torches.

Make a dirt bridge across every canal. This is so grass will spread to every section of the spawn room. Dig a staircase up to the surface, to a grassy area. Dig out the floor of the staircase and replace it with dirt. Make sure the staircase is fully lit. That's it! In short, the grass from the surface will spread down underground to your spawn room. This will take an extremely long time, so start work on the next sections of the trap. Once that's done, close off the staircase entrance and remove the dirt bridges across the canals, and the passive mob section is done!

This can also be upgraded in a manner similar to the hostile mob section of the trap. You must decide on this before proceeding to the Squid Trap section.

3rd Stage - Squid Trap
Squid require one thing to spawn - water, so very little has to be done for this. Dig out a series of canals, the longer they are, the more squid you get. Make all canals lead to a central 4x4 hole where the water stops precisely at the edge of the hole. The hole should be right above the central pit in the lower layer.

For each 7x2 section of the canals, go dig in 8 blocks, and up 4, with a width of 2. Make sure the floor of these sections are 1 above the actual canal. Place signs or ladders in the 3rd block down, and at the far end of these sections off of the canals, place two water source blocks, so these sections act as extended canals. Fill up the area above each of these canals with water source blocks, and that's it! Retreat down to your collection point, and within a couple of minutes, you should have some ink sacs!

This works because squid spawn in water, as long as they have room. Squid spawn in the water, swim down below the signs and ladders and into the canals, which move them through the central hole, and down into the grinder area.

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Introduction
The piston grinder is an alternative to the lava blade as a method for killing mobs. This tutorial assumes you have a functioning mob production facility.

Requirements:
 * Sticky Piston
 * Piston
 * Bucket (With a lava source and a water source available)
 * Redstone
 * Stone Pressure Plate
 * Cobblestone and Wood Blocks

Building

 * 1) Choose a suitable place for the grinder, from the grinder to the chute carrying mobs from the farm will be 16 blocks. Place 1 block above ground level with a stone pressure plate on. The pressure plate must be stone or drops will activate the system.
 * 2) Build a canal 7 blocks long leading to the block with the pressure plate on. If the pressure plate block is 1 high the canal should be at a height of 2, so mobs travelling down the canal will drop onto the pressure plate. The canal should be an 7*1 rectangle with the exception of the 2 blocks furthest away from the pressure plate block. Here the canal should be 2 wide (a 2*2 square). Piston-Grinder-1.jpg
 * 3) The next step is to install a lava blade to deal with spiders. A lava blade should poke out into one corner of the 2*2 square out of the main canal. This corner is shown in the image below. Build a canal 3 blocks long leading back from the corner the blade will poke out into. Place a sign under where the lava will protrude.
 * 4) Place the water source in the block behind the lava blade corner, so it flows round the lava blade and down the canal.Piston-Grinder-2.jpg
 * 5) Now place a wired sticky piston 2 blocks to the left of the pressure plate, 2 blocks up. To clarify, if standing on the pressure plate facing the canal, the sticky piston should be 2 blocks to the left of your head, there should be 1 block gap between your head and the piston. Connect this piston to the pressure plate using redstone wires. Piston-Grinder-3.jpg
 * 6) Next place a block of any type, here I use wood, on the plate of the sticky piston, this is the crushing block and should extend above the pressure plate when powered. Also add a block opposite the pressure plate at head height to trap the mob in the extended block. Piston-Grinder-4.jpg
 * 7) Finally for the sticky piston, add 3 blocks across the front of the pressure plate space at head height to stop the mobs moving forward, the pressure plate block should now have a 1 block space at head height. To clarify, the 3 blocks added stick 1 further away from the canal than the crushing block.Piston-Grinder-5.jpg
 * 8) The next stage is to add a (normal) piston to extend from above to stop waiting mobs climbing over the trap. from the point of view of a mob on the pressure plate, the piston should extend to just above the head, make sure there's space for the mob to enter the trap from the canal when relaxed, so the piston should 4 above the level of the pressure plate. Piston-Grinder-6.jpg
 * 9) Wire this piston up by joining it at a junction with the wire from the pressure plate and to the sticky piston. Piston-Grinder-7.jpg
 * 10) Finally add the lava for the lava blade and box the whole contraption in, and connect to the mob farm. Remember to put a block directly above the lava blade when you box the grinder in.Piston-Grinder-8.jpgPiston-Grinder-9.jpg

Experience Farms (XP Farm)
The obvious disadvantage to these traps are that you must manually kill each mob. It is easier to make these in a way that only requires you to shoot the mobs in a certain area after they have been collected by means of water channels or a "stacking hole". However, it is far more efficient to use Splash Potions to kill all mobs at the same time while still gaining EXP. (Keep in mind that Instant Damage actually heals Undead mob types, while Instant Health will kill them. The opposite is true for all other Mobs.

Big dark room " trap "

This is not even a trap at all. It is simply a very large hollow structure with no light at all on the inside. Natural caves can be used, or one can be made of obsidian to prevent creepers from messing up the room. If the room is dark, big, and high enough, a lot of mobs will spawn. Be sure to add a door, so you can control when the mobs are let out for you to kill. After 1.2, zombies became able to break doors. One alternative is placing trapdoors since they don't see them the same. Another advantage of this is that mobs can get close without hitting you (yes, even creepers) while you are free to hit them with whatever. Actually, just forget doors. Use a trapdoor.

Drive-thru window
It is also possible to make a small modification to the above 50x50 mob farm so that you kill the monsters instead of the lava. The basic idea is to tweak the water flow so that they all get pushed into one point, in a little 1x2 chamber, keeping just a 1x1 window open at their leg level. You arrive from the other side, so that their legs are at your head level (that way you don't have eye contact and cause them to attack), and finish them off with any sword.

The other fun advantage to this is you quite often get a creeper with a skeleton archer behind him, which gives the possibility of them to dropping a music disc. Once the mobs have enough drops, you can fill the top block, open the bottom one, and collect the whole batch of drops. Just like a drive-thru window.

The main caveat is to make sure the whole mob area is properly flooded. A creeper blowing up in water will not cause any destruction, but if they find a patch of dry area they will wreak havoc.

Using this farm I can get to L17 in about an hour of killing, L40 in a few hours.

Door based trap
This trap is easy enough to build, but requires a large surplus of doors, pressure plates, and fences. Find a long border where mobs appear to cross often, or for instance, around your home to keep mobs out as well. Build a continuous fence around it, or if it is a straight line, make the ends move forward one block. Along the border of the fence place pressure plates. Outside of that, place wooden doors in a position outside of that so that the default position is open to allow mobs into the fence. Make sure that when you stand on the pressure plate the door will shut you in next to the fence. Simply wait for this to fill with mobs. As you find mobs in your trap, shoot them with a bow and arrow from the safety of the fence. after all of the mobs have died, enter the trap yourself and collect their dropped items and experience orbs. To reset it, open the door and leave for the trap to fill again. Using more complex arrangements, one can filter the mobs so they will become trapped in one corner of the trap. Note that this is not effective for Endermen from 1.8 on, because they teleport outside of the fence.

Sky Mob Farming
Sky Mob Farming is essentially the same as a normal mob farm, with 2 block high spawn pads and water canals leading to a central hole. The difference is there is no land for mobs to spawn above Y-128 (128 blocks high), so you can have far more efficient mob farms. Build anywhere from 128 up to Y-240 (240 blocks high, beyond which mobs don't spawn due to a bug), with canals leading to a drop hole. To prevent spiders from spawning, cover your spawner pads with half slabs. Put vines at the base of your hole, with buttons or signs to stop the vines growing down, so mobs that fall from different levels will slow down as they pass through the vines. Then have a 23 block drop to bring the mobs to one heart, and hit them for experience and items! A sample design of this can be found here

Spider Farm
This trap is made up of a jail made up of bars (that a spider can't get through but a player can) water and some water conveyor belts. First surround the spawner in a jail that is about 9 wide and deep and 5 tall with the spawner 1 meter off the ground with a water source flowing out of the jail and into conveyors of water that transfer exp and loot into one easy to get place. Kill the spiders with any weapon (even steak as I found out) and collect the exp. If you are also on a server with an RPG plugin you will get better skills which makes your prize even greater!

Pig Farm
This trap is easy to build if you have access to a pig spawner, which can be obtained by mining any mob spawner with the silk touch enchant (Before 1.9 Pre-Release5 as silk touch does not work in 1.9pre5). To build this farm, you construct a lit room with a pig spawner in the center, where the 8x8x3 area around the lower left corner of the spawner is within the room. The room must have a grass floor for pigs to spawn. Next create deep channels with moving water to carry the pigs away from the spawn room to a separate holding area. It is recommended to make the channels two blocks wide to avoid pigs walking over them, and at least 2 deep to avoid pigs leaping out. These channels should focus into one small cell where pigs can be easily killed. This holding area can be of any size, but it must be outside of the 17x17x9 block around the spawner's lower left corner for the spawner to not detect the pigs. This is important as the spawner will stop spawning if there are 6 or more pigs in this block. As pigs are carried from the main room to the holding cell, the mob spawner continues to produce pigs since it cannot detect pigs in the distant cell. The player must remain within 16 blocks of the spawner for it to continue spawning, so it is required that the player wait in the room for some time while the spawner goes to work. As the room is well lit, the player can leave the game idle for hours while pigs are spawned and collected. Collection of the experience is done by killing the pigs manually, such as with a sword. Perhaps the easiest way to do this is to hold the pigs in a 1x1x2 holding cell with an open top, where the player can simply look down into the cell and hack away at the pigs.

Notes:
 * Multiple pig spawners can be used to make pigs spawn more quickly. Simply placing them next to each other can increase speed dramatically. However, they are still limited by the amount of pigs in the region, so at some point additional adjacent spawners do not help.
 * Due to the number of pigs being killed, sword durability and hunger quickly become an issue. Having many swords available and a large supply of food is requisite, although pigs drop far more food than is required to kill them so this immediately stops being an issue.
 * A fast computer is required to handle several hundred (or thousand) pigs. It is not recommended to let pigs spawn for excessive amounts of time before collection to avoid crashes.
 * While pigs yield the least experience, they are perhaps the easiest to farm experience from because they do not fight back.
 * Minecarts can also be used to transport pigs from the spawn area to the holding cell.

Death-pit Experience Farming
Another easy way to farm experience is to use a regular spawning area but replace the regular method of killing mobs with a pitfall-type death chamber. If the spawning area is designed correctly, mobs can be carried out from the collection point and raised up a water elevator (including signs so they don't drown). At the top, there should be a pit deep enough to bring the mobs down to one half-heart of health(22 blocks for most hostile mobs) with a one block hole in the side. This allows the player to reach in and punch the mobs, instantly killing them. If you are good with redstone, it is possible to make a piston gate that can divert the mobs to either the experience grinder or an ordinary lava grinder, allowing drops to be collected when you are busy. This is the most efficient way of grinding EXP when combined with a Dungeon or placed Mob Spawner.

Dungeon Experience Farming
This trap can move a constant amount of skeletons or zombies to a single block and then have them all killed by a splash potion, getting tons of experience without wasting your sword. (There are many ways to make this type of farm more efficient, such as utilizing the maximum spawning space of the spawner and lowering the mobs' health to a point where you can kill them with one punch)

1. Find a skeleton/zombie dungeon. 2. Dig a U-shaped canal in which is 17 blocks long. At the end, put a water source. This works well with a 5x7 skeleton/zombie dungeon. 3. Dig a hole in the middle of the canal in the dry spot. Make it 10-13 blocks deep. 4. At the bottom of the hole, make a small room and have a staircase leading up to the ground so you can access the trap. 5. Where the hole is, place a block on the floor below it. This way, skeletons will clog up this area but not see you. 6. Create some Potions of Poison or of Instant Health splash potions. (Since zombies/skeletons are undead, hitting them with an instant health splash potion will kill them) 7. Wait in the room for a long time and watch as the skeletons/zombies pile up. 8. When you think you've got enough, throw the splash potions at the skeletons/zombies until they're all dead. 9. A massive flow of XP will come out of those skeletons/zombies. Enjoy the repeated popping sound. Repeat this for tons of XP.

Note: monsters cannot despawn when very close to you, so this can make the total mob count exceed 200 mobs. Done with a double dungeon to test this, and the mob count just went up even after eight hours.

Dungeon mob farming can be made even simpler by making 2 wide and 2 deep canals around the skeleton/zombie spawner that lead to a water elevator (with signs so the monsters don't drown) build the elevator so that it will drop the monsters 22 blocks. This is important because if they fall 22 blocks they will live but with only 1/2 of a heart so they die at one hit so you do not have to use a potion to kill them. If the fall is greater than 22 blocks it will kill them and they will drop no XP

You've captured this perfectly. Thanks for tiakng the time!