Knights Onrush from Chillingo and MoreGames Entertainment is a castle defense game that just recently made its way onto the Apple App Store. The goal here is simple: keep the invaders from breaching your castle walls for as long as possible.
There's a variety of defensive options available in Knights Onrush as shown in the short trailer video below. You can use fireballs and freeze spells, hurl giant boulders at your attackers, or even serve 'em up as lunch for a swooping dragon.
Knights Onrush will set you back US 1.99 a pop. It's a bit repetitive, but it does have some nice graphics. A good way of wasting time, I guess, although if you don't mind defending a castle against stick figures you can also try out StickWars for a dollar less.