Every so often you find a game so addicting that you can't stop playing it. I'm that way with two games on the iPhone: Fieldrunners and Civ Revolution. Close runners up are iShoot (there are only so many times you can launch nukes) and now Crush the Castle.
Designed by Armor Games, CtC was originally a Flash game ported to the iPhone.
To play you load up a trebuchet with weapons (rocks, firebombs, whatever else) by tapping once. You tap again launch and then tap to release at some point in the arc. The items swing out into space and land at some point on a castle that is essentially made of beams. The beams react in a naturalistic way meaning they move as if they were real beams and you then crush little people underneath them. Rinse. Repeat.