Robo Defense is the ultimate portable tower defense experience. Featuring open maps, upgrades, achievements and nice graphics. The full version features 5 maps with unlimited upgrades and difficulty levels.
A free version is also available (try it first!).
Free version progress is automatically imported into the full version, should you choose to upgrade.
HTC Desire, San Francisco: Turning off "High Quality Graphics" fixes known issues.
Galaxy S: Turn on "16-bit Backgrounds" to fix studdering performance issues.
Atrix: Disable sound to improve stability
Performance tips: If the game is running sluggishly, try playing fully "zoomed in". Turning off "High Quality Graphics Mode", in the options menu, will also help on some phone models.