One of my favorite iOS games is Kingdom Rush, a medieval fantasy tower defense game. It's free to play on the Web. I talked about it on Jesse Thorn's Bullseye radio show here, and Jane and I reviewed it on Apps for Kids here. Here's what I said about the game in an earlier post:
The cartoonish art is very appealing, as are the monsters and towers. The goal of the game, like all tower defense games, is to prevent the invading hordes from making it through a gate to your kingdom at one end of the display. You do this by placing towers staffed with archers, knights, magicians, and cannoneers along the path that the monsters run down (the monsters appear from a trail emanating on the opposite side of the display). As you kill the monsters, you collect gold, which can be used to buy more towers. Even though there are a few more bells and whistles, it's a simple game -- but addictive.