Looking for a nice tower defense game? Plants vs zombies should be your pick. PVZ can be downloaded onto your computer, or you can install it onto your itouch.
Object of game
So you live in this house, and countless zombies are trying to get in and eat your brains. Your only defense is to plant special plants that will kill the undead zombies. Sunlight is your currency, and you obtain it by:
-It falls from the sky (Day-Time level only)
-Planting a sunflower or sun-Shroom
Sunflowers/aground produce 25 sunlight per 20 seconds, and you use the light to build plants. For example, the pea-shooter shoots peas at zombies, and the star-Fruit shoots fruits in give different directions.
Picture 1
[click on the link, the picture is WAY too big to be fitted.]
Zombies/plants
As you complete each level, you recieve a new plant, and maybe a new zombie to encounter! Here is a short list of some zombies:

-Regular Zombie: Low health, and it's pretty oridnary. It still wants brains.

-Conehead Zombieouble the health of a regular zombie. It likes cones!

-Pole-Vaulting Zombieole vaults over the first plant it encounters, regular health.

Picture 2

Along the way, you're crazy neighbor opens up a shop. here you can purchase plant upgrades, more plant slots, and extra defences.

So overall, I give this game a 8/10. The computer version has more things to do, like more minigames, the Zen Garden, etc.
But for the mobile version, it's pretty sweet. I highly recommend you get this game!