I built a Underground castle with a way point but it's Unlikely to enter from that way with like 20 creepers in a room and zombies.Seems like you're a fellow griefer :D
I just use a little thing called digbing down cause the tip's are were I fall and nothing spawn's in it so far unless I'm gonna add battle basement room's.
My best creation would be "The House" I fill/hide TNT under a house and in between walls and I steal the poor sucker's stuff neat trick to get anyone's stuff (make the house full of chest's a bed crafting tables and make it look realistic)
I made a mob zoo with every single mob exept for slimes and ender dragons . Its pretty freaking awesome.