For the last week or so I've been making my way through the game Eufloria on my iPad. I tend to think of Eufloria as a sort of two sided tower defense game, wherein you are both building your defenses while invading your enemy. There are two types of towers, called trees in this game, one which produces "Spores", the basic fighting unit in the game, and one which produces explosive pods that heat seek enemy spores. The goal is to colonize all the asteroids in a given area by defeating the enemy spores and infecting the soil of their asteroid with your own spores until you take control. There are some extras on top of this basic idea, but that should be enough to give you a good idea.
It's a fun game. You're allowed to play at whatever pace makes you comfortable. If things start to feel too slow, the creators of the game have given you the ability to toggle between three speed settings.
I had a really fun time working my way through the story mode during my commutes to and from work. The graphics are very well designed, and the levels were challenging, varied and required various strategies to win.
If you have a chance, I recommend giving it a try.