or we could build a giant labrynth, centered around a natural mob spawner, so we have a labrynth filled with monsters, and made of obsidian, so creepers don't blow it up. maybe as a sort of initiation? MUAHAHA!
or, we could build a series of underwater domes,(glass) with tunnels 1 block high, with minecart tracks, since you don't suffocate in glass blocks, and it offers great visibility
fianlly, just a castle with large tacle spires on the corners, serving as guard towers.