I designed a cube that will constantly regenerate itself if blocks are broken. It uses a redstone pulser and many, many pistons in unison with a cobblestone generator (water+lava). It has switches on each of the eight pulsers so you can have certain ones active and others not to create a different pattern eg. not generating one wall of the cube. I built a sort-of miniature house to demonstrate a simple use, you could use it for a jail, trap, house or even an un-greifable admin base.