No one likes it when random people wander into your home and mess things up, which is why it behooves you to hide your things from even the most prying eyes with builds like this totally invisible, 2x2, piston-operated door with no visible redstone either inside or outside. The story of Goldilocks and the Three Bears may have gone differently if they would have had one of these babies installed in their home:
I did a tutorial for a piston-powered trapdoor and many people wanted to know how to get the same functionality in a horizontal, rather than vertical, door. So, here's how:
Step 1 Pistons!
Put down 8 sticky pistons, four on each side, facing inward. There should be four empty spaces between them.
Step 2 More Pistons!
Now add four more pistons, two on each side, facing whichever way you want the entrance to be.
Step 3 Give Them Butts
Half of your pistons need butts. Give them butts. Add two blocks to each side, with the bottom block toward the front and the top block toward the back.
Step 4 The Roof Is On Fire!
Okay, the roof isn't on fire, but the roof is on. Cover your pistons, and make a roof across the top of your passage.
Step 5 Repeaters
Time to add some repeaters. Add one on each side, facing toward the back of your door.
Then add four to the roof, leaving a 2-by-2 space in the middle. These repeaters should be facing away from the middle, and the two in the back should be set on the last delay setting. The two in the front should have no extra delay.
Step 6 Wire It Up!
You're almost done! I bet you can nearly taste that sweet, sweet piston action. Wire that bad boy up! You're essentially just putting redstone on every square that you can see from above, which should be 12 squares of redstone altogether.
Step 7 Add your switch and you're done!
Your switch needs to attach to the four-block redstone square in the middle of the roof of your design.
So, you can really do whatever you'd like at this stage in order to connect it to where you want it, but I made some little stairs to the side for simplicity's sake, and then wired it up like so:
BAM, done! Now you can just insert this mechanism into any pre-existing wall (or build one around it) and you're good to go!
Step 8 There's More Than One Way to Skin a Door
In case you have trouble making your door flush with the surrounding wall, here's what you do. Turn the door on, so that your pistons are all fully extended.
Cover it up with whatever you want it to be covered by (any other block type will work).
This is what it should look like when you're done. Just a flat, featureless wall... Until you pull the switch!
Then POW! Door out of nowhere!
This is a really great design, pioneered by our very own member nicholas1413 (aka SilentCircuitry on YouTube, check out his channel!). It's great because none of the redstone will show if you don't want it to, and you can incorporate this mechanism into any wall where you have three spaces on each side, and two spaces above.
Now go hide things!