Tutorials/Monster spawner traps

Spawner traps are a type of mob farm that uses a spawner. They employ the mob spawners found in dungeons as a source of mobs. The purpose of these traps is to create an infinite source of items that can be gathered in an efficient way from the mobs spawned by the monster spawner. Two general goals of these traps are to keep the player safe and to allow the mobs to spawn.

Finding dungeons
Dungeons are found when tunneling or exploring caves and mineshafts. They appear to spawn in random places, but almost always underground, usually at the Y=30 to Y=60 range. Finding dungeons without mods, cheats, or external programs is mostly attributed to luck and some know-how. While exploring, some tricks can be used to help detect dungeons. Gravity blocks like sand and gravel in a neat square may cover a spawner and is easily visible in deserts if spawned near the surface. Hearing or seeing a group of single-type mobs while caving could mean a spawner is nearby. Lastly, mossy cobblestone or normal cobblestone naturally spawning away from lava, is a dungeon, unless it is in a jungle biome, where it can also be a jungle temple.

Mob spawners can be neutralized by placing torches to light up any dark areas around the spawner. With the exception of blaze spawners, a single torch placed on the spawner is sufficient to disable it. Because blazes can spawn up to light level 11, multiple torches (light level 14) are required to neutralize a blaze spawner. There are six different mobs that can spawn. They are separated into two categories: the two-block high tall mobs - zombies, skeletons, and blazes - and the one block high short mobs - spiders, cave spiders, and silverfish. Similar variants such as the zombie villager are not included in this list.

In the Nether, blaze spawners can be generated in nether fortresses, and can be trapped to collect blaze rods and mass amounts of experience. Blaze spawners are the most dangerous to trap because of the height of the structure and the additional danger of lava. One must be well-prepared to contain this type of spawner, lest they might die from heights, lava, the blazes, or even external mobs.

Silverfish spawners can be found in the portal room of a stronghold. However, silverfish don't drop anything and are bothersome unless properly trapped. You may choose to simply break the spawner, unless you want to create a silverfish trap near an entrance to your base. Silverfish have some advantages: they are the easiest hostile mob to kill.

Normal spider spawners are one of the easiest to trap, as they cannot fit through a 1 wide hole, so you can dig through the side and place a torch on the spawner without fear of getting hurt.

Cave spider spawners are hard to trap since you will get poisoned on normal or higher difficulty, novice and intermediate players may be better off breaking or disabling these as soon as possible. These spawners are generated more often than other types, and some may even generate within 20 blocks of one another, which can be leveraged into an efficient farm. Because cave spiders can fit through 1×1 spaces and between slabs, it is possible to make small, simple, and efficient grinders, although the difficulty in containing cave spiders can lead to spiders leaking out and/or poisoning the players.

Zombie spawners are also a bit tricky, as the spawner may spawn baby zombies, which are a 1×1 mob, making proper containment essential, (particularly on harder difficulties). The zombies may spawn other zombies nearby to "help", making area security a priority, and the spawner may also spawn zombie villagers which makes adding in some manner of player-controlled sorting mechanism a consideration. On that note, however, zombie spawners can prove to be one of the most lucrative to trap, as the variety of items it can drop is extensive, and potentially limitless if you factor in the added option to use it to generate Villagers, which can then, in turn, be traded with, themselves "farmed", or used to make iron golem farms.

Simple farms
The three most common types of spawner are for zombies, skeletons, and spiders. In each case, it's fairly trivial to set them up so that you can attack the monsters without retaliation, forming a simple farm for XP and their drops. Sometimes cave spider spawners can be treated similarly, but this is more hazardous. Proceed as follows:
 * 1) Secure and light the area around the dungeon. Light the room and spawner while you work on it, so the monsters don't interrupt you.
 * 2) Ransack and clear out the chests. Choose and prepare a "front" area for the farm, with a chest and crafting table to supply swords. Make sure you have a clear approach and/or room to back away quickly, in case you come by and find one or more creepers spawned inside, and you need to shoot them. The details of the front will vary according to what monsters you are farming (see below).
 * 3) Prepare the spawner area by digging out the entire spawning range. That's a 10x10x4 area centered on the spawner. So you'll dig two layers of the floor underneath, two blocks above the spawner, along with walls to five blocks out horizontally. In order to maximize spawn rates, however, in case it's not a spider spawner, it is advisable to dig a further 5 blocks under the spawner, dropping any non-spider mobs outside of the spawner's detection range. Fill in the walls surrounding the farm, except at the front. Close off any extra exits and alcoves, fill stray holes, place fences as needed.
 * 4) Set up any Redstone machinery, including access tunnels and alcoves.
 * 5) Prepare the front according to the monster type (see below). You will always want fence gates, in case you want to dash in between spawns to collect drops, or to fix something up.
 * 6) Most farms will want a flood system, to wash monsters and stray drops toward the front. If you've opened up the full 10x10 area under the spawner, you can get a nice sheet of water from three water source blocks at the back. This will push monsters forward and bring all drops within reach of the front. If you still have items being stuck in one corner, you might resort to a side tunnel and/or a sticky piston to get at them without exposing yourself.
 * 7) Once your farm is prepared, remove most or all of the lights. Beat a hasty retreat to the front.
 * 8) Make some swords and whack away. Keep bows and arrows (or tridents with the Loyalty enchantment) handy near the entrance, in case a creeper spawns while you're away or approaching, or otherwise sneaks into the area (normal darkness spawns are suppressed within 24 blocks of a player).

A bit of a special case: if you found a dungeon on Mushroom Fields or Mushroom Field Shore biomes, congratulations! Proceed as usual, but without worry about creepers or other stray monsters intruding.

The Front
The setup at the front will vary according to which monster you are farming:
 * Zombies: a simple gated fence will do. Maximize your view of the spawner, so that they come to the fence quickly.
 * Spiders: cover the front with pillars of blocks, floor to ceiling, but with 1-block wide slits open between them. Some glass at the ceiling can keep the spiders from sticking up there out of reach. You can then attack them freely, but they can't come through. To defend against creepers and other non-spider spawns, you can make these slits two-high, between the top and bottom slabs (that is, three blocks total, but with the top and bottom spaces half-blocked by slabs). The "tall" monsters will not recognize these as passable, but you can move or attack through them easily (or you can just put fences and gates to block walking monsters).
 * Cave spiders: you can place a door between floor-to-ceiling fences (see pictures below). Iron doors are better so that you can't accidentally open them while you're attacking the spiders. Note that cave spider spawners don't come pre-enclosed in a dungeon, so you'll likely need to start by building a room around them. Remember to open up the entire 9x9x3 area so that you don't get snuck up on by spiders spawning in a way that ignores walls. Again, glass can help keep them from climbing out of reach.
 * Skeletons: are the trickiest, as they can shoot through even brief gaps the moment they get the angle. For the actual killing zone, you want fences/gates at eye level (so they can't shoot you), with an open row below (so you can attack). If you stand directly in front of the fence, they will usually come up to the front so you can attack them. That row in front of the fence can also be a trench—you can reach further into the room, but then they can occasionally get an arrow or two under the fences.

Redstone and other contraptions

 * 1) The most common use of Redstone is to switch a flood system on and off. Instead of placing the water blocks yourself, embed dispensers in the floor or wall, each containing a bucket of water. You will need Redstone dust and switches to power these, but remember that flowing water will wash away Redstone and switches.
 * 2) If you're ambitious, you can build a system to light up the dungeon when you flick a switch. This will be much easier if you have redstone lamps, but it can even be done with blocks pulled by pistons to uncover jack-o-lanterns.
 * 3) You can also convert the front to feed monsters into a killing device as discussed below. If they are killed without player intervention, you will not get all of the drops, nor experience, but this can still be useful for the drops you do get. You can also tune the device to soften up the monsters before killing them yourself.

Killing mechanism
Any standard mob grinder can be connected to a dungeon. However, the purpose of this device is for players who are currently low on materials and can't yet build something larger. A hopper can be used to collect the items into a chest.

Lava blade traps
A lava blade trap consists of using water underneath the spawner and funneling these into a single line. At this point, the mob should find themselves approaching a sign at ground level and a 'lava blade' above it which flows over the top of the sign, but no further. This allows the mob to drop items at their feet for your collection. A tutorial can be found here: .

Timed lava dispenser


A variant to the classic lava blade, the timed lava dispenser can be used to damage a zombie or skeleton to the point where one hit can kill it, allowing the player to get advanced drops and XP.


 * 1) The mob arrives by water stream through a two block high, one block wide tunnel, and is pushed up against a Nether Brick Fence or Cobblestone Wall).
 * 2) A dispenser is placed to dispense lava on top of this fence post, while a sign prevents the lava from flowing into the tunnel.
 * 3) A button is linked to a redstone timing mechanism that will activate the dispenser twice; once right away when the button is pressed, and then a second time a few seconds later to retract the lava.
 * 4) Since the mob is halfway on the block with the fence post, it will be damaged by the lava, but since its feet are still in water, the damage stops exactly at the moment the lava is retracted.
 * 5) Hoppers under the fence post and the block before that in the tunnel can catch all dropped items.

The timing in the repeaters will have to be adjusted depending on the mob type.

Drowning traps
Drowning traps employ water to drown the hostile mobs. Mobs and items float in water. In this example, the dungeon is a 5×5 spider spawner, since drowning traps don't work on zombies and skeletons.
 * 1) Capture the spawner and place water in one of the corners. The opposite corner should have one dry block.
 * 2) Dig a small hole in the dry corner. This is where the water channel is recommended.
 * 3) Place two signs in the corners and one in the hole. Water considers signs to be solid blocks.
 * 4) When water is placed in the corner, it will be held in place, even though it will look like it is flowing backward.
 * 5) Place Glass blocks in the corner to create the drowning chamber. Any solid block works. Glass panes and iron bars have a space of air that the mobs can breathe in, rendering the trap useless.
 * 6) Fill the chamber with water to the ceiling.
 * 7) Close up the chamber.
 * 8) Place a water stream at the top of the chamber to push the items.
 * 9) Place a top slab in the water source. It should get waterlogged, so it will still drown the mobs and also push the items.

Take out the torches and enclose the dungeon. Use signs to hold up water.

Zombies and skeletons will sink instead of float, and will not drown. This new feature provides other opportunities, however; for example, harvesting drowned in Bedrock Edition is a source of tridents and nautilus shells.

Please note that the below tutorial is slightly imperfect; the glass panes have air spaces, sometimes letting the mobs survive. Use glass blocks instead, to prevent this.

Please note that you must fill up the column using source blocks, not just one at the top.

Flow trap
This trap is highly recommended. It's effective and safe on all mobs, except blazes, since you need water. Disable the spawner, and then make a 10×10 room around the spawner. On one side, place water on every block. On the opposite side, make a one-block deep trench extending along the side opposite the water. Light up the trench well. The holes have to be next to water. Now, get plenty of food, a few chests, and two or three swords. Remove the torches and let mobs spawn. The water will bring them to you, and you can shoot them and hit them. Also, make a room to eat and heal in safety, just in case you didn't build the trap thoroughly. With this trap, you can go for hours in one go, and gain a massive amount of experience levels and items at minimum risk. The trap system is built in a way where you can easily leave when you want, with almost no creepers or non-spawner mobs popping up.

If the spawner is a spider, make these adjustments:
 * Add iron doors with levers to control mob flooding and possible death.
 * Bring milk for cave spiders.
 * Add an on/off lighting system.
 * Bring bows to help control mob flooding and possible death.

Sunlight based traps
This type only applies to zombie and skeleton mob spawners. Sunlight-based traps are made by placing water below a spawner. This water will flow from one side to another. Where the water stops, dig a pit a few levels down. This will drop the mobs into the pit. To finish, make sure that the pit is open to sunlight. This sunlight will burn the mobs alive. An example of this can be found here:.

Potion-based trap
This is a fairly expensive and hard-to-make trap, so it's not recommended.

First: Light up the room to prevent mobs from spawning.

Second: On top, dig out 2 blocks from the spawner room top without breaking the spawner roof Third: Place the wires and dispensers like shown:

DWWWWWD   X - Roof Block XXXWXXX   W - Wires XXXWXXX   D - Dispenser looking in the spawner room XXXLXXX   L - Lever, button or even a redstone clock, whatever you want to power XXXWXXX       the dispensers up! XXXWXXX DWWWWWD

Fourth: Put inside the dispensers:

HARMING POTIONS: Used to damage spiders. HEALING POTIONS: Used to damage skeletons and zombies. POISON POTIONS: Used to weaken any mob that is not undead.

Fifth and last, but not least: Take the lights out, get up, and when you hear the mob noises, push the lever/to activate clock/ whatever you used to power it up.

Iron golem traps
In this trap an iron golem is used to kill the monsters for you. To make it, simply make an iron golem in the dungeon and watch it kill for you. However, the iron golem will eventually die (especially if more than 1 mob is spawned), so you'll have to replace it with another, or link it to an iron golem farm.

Wolf traps
Similarly to the Iron Golem trap, have tamed wolves kill mobs for you. However, this only works for skeleton dungeons, as wolves will only automatically target skeletons. Additionally, as wolves have much less health than golems, you'll need to either have many wolves available, or weaken the skeletons beforehand (i.e. a large fall). The benefit of wolf traps over golem traps is that kills from tamed wolves can produce experience and extra loot.

Cactus traps
The following cactus trap is designed for spiders. It has a chance of destroying the items, but this is an inherent issue with cacti (this can also incorporate a small cactus farm by placing a block 2 blocks above the cacti).


 * 1) Dig the main channel. The spider spawner is left in the wall forcing spiders to spawn on the small dry patch.
 * 2) Dig two blocks down and one block in just before the cacti. These two holes will hold the collection water sources.
 * 3) Place three ladders on the far side of the collection output. Place one on either piece of sand. Place the two water sources in the holes.
 * 4) Place the main water in the two openings. The block between them makes the water flow towards the gap in the cacti, slightly improving the collection rate of string.
 * 5) Cover the entire trap and remove all lights.

Fall damage
Fall damage onto hoppers is another option for killing mobs and collecting items automatically.

Villager traps
This trap only works with zombies. Both rooms in this design must be very dark. The zombies will try to get the villagers, they will fall through a hole, and if the room was high enough, kill themselves in doing so!


 * 1) First make a room high above the ground and move at least 2 villagers to it.
 * 2) Now put at least nine doors in it.
 * 3) Next change one of the walls to glass.
 * 4) Attach another room to the side that the glass wall is on.
 * 5) Mine a hole in the floor of the empty room.
 * 6) Line the hole with open trap doors (if your hole is 2 by 2 you will need 4 trap doors)
 * 7) Directly under the hole, make a room without a roof on the ground.

Blaze traps
One way to make a blaze farm is to box in the spawner, so that the blazes can't fly away. Leave a space at the blazes foot level and stand so that your eye level is at that space. That way the blazes can't hit you (mostly), but you can hit them. This is also a pretty good experience farm since each blaze drops twice as much xp as other mobs with its health. Be warned, however, that sometimes blazes will spawn outside the box so it is advised that one box themselves in as well.

Alternatively, see Tutorials/Blaze farming for video tutorials.

A note on lava
Lava can quickly kill mobs and can be suspended on either a ladder or a sign. To suspend the lava, create a 3-long channel then put a sign or ladder on the last block. Lava spreads three blocks away from the source, so the last section of the lava will be left suspended on the sign or ladder.

Lava generates light, which can lower the spawn rate in the dungeon unless placed far away; however the increased turnover of mobs may compensate for this.

"Johnny" Vindicator Trap
Since Java Edition 1.11, you can use an anvil, nametag and a vindicator to make a monster spawner trap. First, you need to find a spawner, then you need to make a wall with a door to make no monsters to leave the trap, then lure a vindicator to inside the trap.

Then you go to your anvil and rename the nametag to "Johnny", then right-click the vindicator with the name tag. After it is renamed, the vindicator will attack any other mob except other illagers

It's recommended to put more vindicators inside the trap. and also it's recommended to heal your vindicator using Potions of Healing.

Magma trap
Magma blocks can be placed around the spawner as a floor, allowing mobs to spawn and then killing them. Items can be collected via minecart with hopper running below the floor, or with water leading to hoppers, making the trap fully automatic.

Other
Traps that use unusual methods, such as huge pits, fall into this category.

Note: Using a slab of cobble, planks, or sandstone in murder holes will prevent creepers and skeletons from seeing you, then you can retrieve items without breaking blocks, place the slab on top of where the mobs would usually stand.

Note: Water mob elevators no longer work on zombies or skeletons, as they now walk along the bottom of water. They will still rise up bubble columns created by soul sand, but the challenge is getting them to enter the column.

Pit trap
For this method, you must dig a pit that reaches far under the dungeon. Depending on the height (at least 23 blocks), the mob will die on impact, as long as there aren't any blocks of water at the bottom and the mob has weak enough or no armor. However, this method takes a long time to make and only works for spawners near or above the surface. Alternatively, you can build an elevator for the mobs using alternating water and signs to bring the mobs 23 or more blocks higher, and then drop them 23 or more blocks in order to kill them (or you can do 22 blocks if being used for an XP farm). Keep in mind that skeletons and zombies can randomly spawn with armor, which can be enchanted randomly. This can be remedied by placing a platform of magma blocks at the bottom.

Experience trap
For this trap, you need to dig a large pit 22 blocks deep under the dungeon, or make a water elevator taking them upwards so you can stay near the monster spawner. Go to the bottom of the pit and place water in a fashion so that it should flow towards a wall. Then make a room where the water is flowing. This is where you will stay while finishing off the mobs. Make the room so that it has a one-block hole where you can see the mob drop zone. Hopefully, mobs should spawn, fall down the pit and survive with half a heart, get pushed towards you by the water current, and get killed by a single hit. If you like, you can bring food along as your hunger will deplete over time. This should provide normal as well as rare mob drops and experience levels, making it quite useful. Another technique is to leave your player by the dungeon for a couple of hours, overnight is suggested, and there will be at least a hundred monsters waiting to be harvested. Keep in mind that the maxEntityCramming gamerule can limit the output.

Efficient pit + experience trap (by Epiiik)

 * This grinder does not work in 1.13+, since skeletons and zombies no longer swim. However, it is possible to build a similar grinder using soul sand block bubbles to bring mobs to the top of the water column. See Tutorials/Drowned farming for an alternative simpler design.
 * Note, all dimensions are given the format: Horizontal x Vertical. This method is an indirect hybrid of the aforementioned traps, utilizing both fall damage and mob control with water. In the mob spawning room, dig 1 block down so that the spawner is at eye level, and place water blocks at two adjacent corners. On the wall opposite of the two water blocks, dig a 1×2 hole for three blocks in the middle of the wall (allowing mobs to pass through).

Begin digging upwards in a 1×2 fashion for 24 blocks. To get up, place signs on the long-faced wall (your left/right) in a zig-zag pattern going up while placing water blocks in between the signs. The first sign should be placed as furthest, lowest space possible. Stop placing signs after the 23rd level. The last water block should be against the wall furthest away from the mob spawner on the 24th level.

On the 25-24 block levels, dig a 1×2 hole for seven blocks in the direction the mob spawner. On the seventh block in, dig one block downwards. Dig in the forward direction for eight more blocks (again, 1×2). You should notice the water block placed on the 24th level should stop where the path drops by one block. Place a water block against the wall of the drop. Notice how this water block stops 1 block short of the path.

Dig further for two block and begin to dig downwards in a 1×1 fashion for 22 blocks, Face the direction of the mob spawner and dig another 1×2 hole one block deep. Dig down one and fill the top level with one block so that you can only see through one block of the pit you just created (if you are using a skeleton spawner, you need to stand one block away from this hole so that the skeletons cannot shoot you). You are now standing at the loot point. Now that the almost-dead mobs are depositing into your 1×1 hole, simply attack them with any item to gain their loot and experience! You may use TNT + water to do splash damage to kill all mobs at once at the risk of lag spikes. Splash potions of Healing also work, as they do damage to most hostile mobs.

Note: you must have 2 block tall non-exploding mobs to do this or the mob either destroys or escapes from the trap.

Useful tips on this design by Duck//Marvin: When you make the opening in the spawn chamber for the mobs to exit, make it 3 blocks high and place a sign or slab at the top. This lets the mobs into the elevator more easily and prevents water spillage from the elevator. Also, making the transport corridors at the top 3 blocks tall instead of 2 allows the mobs to be transported faster as they do not hit the ceiling when they try to swim and get stuck.



Alternative layout
An alternative layout with a smaller footprint takes advantage of the fact that a player must be within 16 blocks of a spawner for it to activate, but that spawners only check 9 blocks vertically for spawned mobs. Build the trap as above, except instead of being off to the side, the killing floor is 4-16 squares directly above the monster spawner. This requires less material, less x,z space and less precision in construction, though is 3-15 blocks taller.

To construct the trap this way, build a 9×5×9 room around the monster spawner with flowing water leading the mobs into a corner. Build a water ladder in this corner going 26-38 blocks upwards as measured from the top of the monster spawner. At the top of the ladder, direct the mobs a couple blocks in any direction with water, and into a 22 block drop chute. At the bottom of the drop chute, create a platform 1 block lower, with a 1x1 "window" into the chute through which you can strike the mobs safely from the platform.

Another experience trap (by AlexxShadenk777)
Firstly it should be noted that this was made with a zombie spawner in mind. Basically you just make a balcony of sorts and wail on the zombies as they approach you while being unable to hit you.

Seal the zombie spawner in a 5×5×5 chamber, naturally, you want it to stay in the center of the chamber at the lowest height. Outside of the chamber make a platform that is 2 blocks high that surrounds the outer wall, and while standing on this platform, punch a wide hole in the corner of the chamber 1 block above the platform. Remove the corner block so you can stand in the corner while the zombies practically funnel toward you. They'll be able to push you, but not hit you at all, so you're free to smack them on the head with your weapon of choice.

Naturally the rotten meat they drop will remain inside the chamber, but the experience points will hover toward you. If for some reason, the points end up too far just walk around the chamber to their nearest corner.

Optional: lead some tamed wolves into the chamber before sealing it!

Experience trap

 * 1) Place torches to turn off the spawner.
 * 2) Place water in two corners.
 * 3) Dig a tunnel where the water converges.
 * 4) Dig the tunnel path until the water ends.
 * 5) Arrange a barrier so that mobs don't make it past the end of the water.

Recollect the torch and stand where the end of the tunnel is.

If done correctly you can slay the mobs and get experience and rare drops from mobs.

Humanoid monster spawner xp trap (by HeofShadow)
This type of spawner is made for Humanoid type monster spawners (skeletons, zombies, and in some cases zombie pigmen), but can be modified for a spider spawner. First disable the monster spawner and areas around it with Torches so Mobs do not spawn and disrupt the creation of the trap. Now make a room 17 blocks wide by 9 blocks long and anywhere from 4-7 blocks high, with the spawner being in the rough center and 1-2 blocks above the ground. Now dig out the bottom row 1 down everywhere except the center, which should be dug down around 7 blocks. Place a water source block on each end of this dug out area and it should stop just before the center. Now place water source blocks at the top so that it should stop before running into the dug out area. Remove the torches so that the mobs start spawning, and go down so that you are at eye level with where the center area was dug down 7 blocks. Make it so you can only see the mobs feet on one side, and on the other use a slab to make it so you can just see the top of its head. This allows good use of both swords And bows (Also, some servers give bonus damage for Headshots).

S-bend design
This type of trap allows a one-hit kill on zombies and skeletons; however, the drops and the experience are farther away.

Expand the floor so that there are two blocks of air below the spawner, and the walls so that the room is 9×9. Dig four blocks down on the block directly below the spawner and place water source blocks in each corner.

Dig one block north and two blocks south (three blocks tall for each direction), then go towards the south end, and make an access shaft. At the north end, dig a shaft, 30 blocks deep, and three pointing west. At the bottom, make a small room and light it up. Place ladders on the west end, and make an access shaft to the top of the ladders. Place blocks to the east of the ladders, except for the bottom few.

Place two bottom slabs on the block south to that directly below the spawner, and place trapdoors liberally (zombies and skeletons can climb ladders). Optionally, place hoppers at the bottom of the collection room.

Multiple spawners of the same species
To maintain maximum efficiency a trap must take on an increased complexity in keeping mobs separated or moving quickly when dealing with multiple spawners of the same species. Minecraft first checks within the 17 wide by 17 long by 9 high area with the spawner in the center (or 8×8×4 radius from the spawner) for mobs of the same species as the spawner. If there are fewer than 6 mobs of the same species in the checking zone, it will spawn a mob within half the distance of the checking radius. This means that with two spawners of the same species near to each other, there will be a zone that mobs will count for both spawners and severely impact the spawn rates. It would be a wise idea to work out where these checking zones overlap and ensure that mobs being funneled to the trap do not enter that zone and/or leave the overlapped / checking area as quickly as possible to maintain peak monster spawner output.

Videos
This design is easy to build and doesn't require too many materials. It allows mobs to enter, but blocks them from leaving with pressure plates and pistons.

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