Did you know that MMO maker NCsoft made an iOS game? Gem Keeper came out late last year, and it's a pretty nice little tower defense game for iOS devices. Today it's on the App Store as the Free App of the Day, so consider this a heads up.
The game has 30 cute levels and about half as many different types of towers. What's unique about Gem Keeper is that you can slide the towers around a track to continue firing on the waves of attackers. After trying this I think you'll agree that all tower defense games should have at least one moving tower from now. It sure beats waiting around for the wave to pass.
Gem Keeper is free for today only, so download it now.