Robo Defense is the ultimate portable tower defense experience. Featuring open maps, upgrades, achievements and nice graphics. The free version features 1 map with 11 difficulty levels.
A full version is also available with more maps and unlimited upgrade and difficulty levels
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: Try the "16-bit backgrounds" option 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.