Pushed up pistons to ward off zombies, skeletons and creepers.
A similar idea already exists using the mechanism as a trap.
If someone (opponent & player) steps on one of the pressure plates, the respective piston is raised and the way is blocked. To prevent that you can simply jump over it, a series of blocks is placed above it.
If the pusher plates are placed inside the circle it is not possible to get out unless you destroy blocks.
In this case, the push plates are placed outside, it is not possible to get in without destroying blocks.
Since NPC's can't intentionally destroy blocks (creepers only explode in the presence of the player) it's not possible for them to get in, unlike the player.
Of course, the blocks are put back in front of the house. I only removed them to show the mechanism.