There are a few tower defense games available in the App Store, but we think we’ve come across the best in TapDefense. Aside from being a free game, it’s really well done. The gameplay progresses in difficulty just as you’d expect it to, and developer LL Group even worked in the iPhone accelerometer into the gameplay - not something you find in your typical tower defense game. One of the towers available is an earthquake tower, and when enemies approach it, you shake your iPhone or iPod touch in order to destroy them. Very nice. In all, you’ve got 42 levels, six tower types, and seven different enemies to combat. And like we said, it’s free.