BackgroundRobo 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!).
note: 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.