Right, so you've made your invincible walls but people have built over them and still managed to raid your base!
Well you can trick all raiders with this completely hidden trapdoor mechanism! You can put stuff you don't care about in chests on the surface, while all your valuables are hidden beneath the floor! Those raiders are going down!
To confuse raiders all you need to do is follow these steps!
First place two sticky pistons one on top of the other facing sideways.
Then get another sticky piston and place it facing upwards next to the other two pistons, then place a block from the floor onto it. Then fill in the rest of the floor leaving a gap for the pistons.
Place a lever on the floor above and then a block with redstone dust on top so that when the lever is activated it will power the redstone. Then place a repeater facing where the piston facing up will be moved. Place MORE repeaters to send a signal along to the two pistons we placed first.
Inspect this picture closely to make sure you are doing it right.
Then flick the lever and it should close! if not re-read these instructions!
Step 5: (optional)
If you want to be able to open/close it from above and below then connect a Toggle Flip-Flop to the redstone which is powered by the lever. by connecting separate buttons to the end of the T flip-flop it will be possible for it to be opened and closed whether you are in it or not! Abracadabra!