Tutorials/Secret door

Secret Painting Door
Start by making a 1x2 gap where you want your doorway to be. Be sure that the wall area around the secret doorway is big enough for the size of Painting you want. For example, for a 2x3 painting, you will need 2 blocks on both sides of the secret doorway, because the doorway counts as 2 blocks. The painting will not stay on the wall unless you follow the next directions carefully: First, place a solid block in the bottom of your doorway, and in the top section place a sign.

To put up the painting in the right spot, look at the lower left-hand corner block of where you want the painting and right-click on that block (hint: you may need to replace the painting a few times to get the right size).

A way to make the doorway easy to enter is by making a 2x2 hole and putting four signs on the side, then use the sides of the signs to put up a 2x2 painting, allowing a bigger hole to walk through.

As this may be a little hard to understand a video tutorial is below: http://www.youtube.com/watch?v=j7caVpkgDY8&feature=

Piston Hidden Door
For a door that is completely hidden except the lever/Pressure Plate/Button to power it, you can use a Piston door. A very simple version is one that just moves 2 blocks to the side with 2 Sticky Pistons, which are hidden inside a wall and are powered by Redstone. To make some more complicated and fancy ones, you can get the blocks to move first away from the Player, and then sideways, allowing for 2 wide doors and it also allows you to build it so that the door appears at the middle of a wall, instead of it only being possible to make it either in a corner or in a tunnel of the desired width of the door, because now all the pistons are behind it. You have to do some redstone wiring, though, and if you want to make the door 2 wide it will need at least 12 sticky pistons, 4 to move the blocks into the wall, 4 to move them sideways, and 4 to move the four pistons that moved the blocks sideways sideways. You have to make the delay from the input to the first four pistons, the ones that move the blocks into the wall, 2 ticks. Then split the redstone wires that go to the other 8 pistons; one way has a delay of 1 tick, the other one a delay of 4 ticks. Then after the delays you can join the wires again and bring them to the pistons. To make it more hidden, replace the blocks with bookcases.

Underwater Hidden Door
Doors and entrances can actually be hidden underwater. This is accomplished via the use of water blocking objects such as trapdoors, ladders, signs etc. Many players prefer to hide these underwater entrances in man-made fountains, bodies of water or behind/in waterfalls. When creating an entrance in the base of a body of water dig a small hole, 1 block only is hidden the best. Then place a ladder at the entrance of the hole, you should see that this will block out the water, leaving an air pocket. You can now continue this ladder shaft down into your base. Be careful when placing torches or light sources to not make this hole easily noticeable.

Lava Door
Minecarts can be clicked on through lava,and your player will instantly be inside the minecart WITHOUT having actually gone through the lava. This can be used to your advantage. First, make your doorway(at least 3 layers of blocks as the door frame. Then place one minecart rail block on the floor outside the doorway directly in front of it, and another inside, directly behind your doorway. On each of these rails, place a minecart. Optionally, half steps may be placed around the rails to keep the minecart from being pushed. Remove one block from the top middle layer of the door way, and remove one directly below this hole on the floor as well. Then get a bucket of lava, and place the lava into the top gap inside the doorway, being careful to move out of the way immediately to avoid getting a faceful of lava. You can now use your Lava Door by clicking through the lava to the minecart on the other side.