This door can be used for hidden bases or storage rooms. It is very fast, and it has a small profile, being two by five blocks and only three high.
Step 1: The Foundation
To start off, clear a space of two blocks by five blocks and three blocks under the stair case that will be modified.
Step 2: The First Layer
The first layer consists of the following blocks: one sticky piston, one dropper, and five blocks of your choice. Add an item of your choice inside of the dropper.
Step 3: Redstone!
Redstone for this step is one repeater set to two ticks, one torch, and three pieces of redstone dust.
Step 4: The Second Layer
One comparator, two piston and one hopper.
Step 5: The Third Layer
Just a few stairs
Step 6: Two More Blocks
Add a block to the back stair, and a half slab next to that block.
Step 7: Put Redstone on It
Add a piece of redstone to the half slab.
Step 8: Power It
Step 9: Finished
Add stairs around the door.
Step 10: Extra Step
This build can be mirrored to make the door two wide.