It's simple, but sometimes simple is good. You destroy castles... There is a bit of strategy to it, sometimes through trial and error you see what hitting it in certain areas does as apposed to others. Made my own here (http://armor.ag/1pmju) but I don't think you can beat it with just rocks... Good game.
I like it so much, here is the one I made!
http://armorgames.com/play/4552/crush -the-castle-players-pack?share_id=523 4f41613fd9d6c893ca7da510d45e3