An inconspicuous secret door in the wall that can be opened with pistons.
You can build that in any wall and not be seen at all.
The downside is that you can then only get a 2×2 pass and you can see the piston when it's open.
Instructions:It's actually quite simple if you already have a bit of experience in Redstone.
Just build the one on the picture on both sides and in between 4 more pistons,which push the blocks forward.
Above and below the pistons, in the middle, set redstone facets, as on the picture.
On the Redstone Fakeln at the lower pistons, you should still put blocks. Then you connect all connections with the levers and ready!
You could also do it with a button, but that would be too complicated.
Important:The pistons on the side should take longer than the ones in the middle. I recommend an extension of 3-4 levels at the Redstone amplifier.