I like tower defense games a lot, but find that most of the popular ones available for Android or iOS are largely unplayable (I love Plants vs. Zombies on my PC and Xbox…on my Android, not so much). GRave Defense HD is the first mobile tower defense game I’ve played that actually does a decent job of using the available screen space so the game doesn’t feel claustrophobic. Placing and upgrading defenses is incredibly intuitive, and the game gets very challenging as the levels progress without feeling like it’s trying to cram too much onto a 4″ screen.
Robo Defense and Defensoid are the best of the lot. There is also another new game that offers similar amazing gameplay with good graphics called Dungeon Defense.
https://market.android.com/details?id=com.dungeond