Tower Madness Zero is this free app on my iPod Touch and then iPhone and then iPad. There are additional paid versions and in-app purchases allow you extra levels and extra weapons. But even without spending a penny, this game will offer you hours and hours of entertainment.
This is your typical tower defense style game -- aliens are invading and they want to steal your sheep. It doesn't explain why and I'm not asking.
Anyhow, so the aliens land at designed areas and then make their way to the sheep. Sometimes the path is wide open, sometimes there are hedges or obstructions or specific paths.
So, like I said, I've gotten a lot of hours of fun out of this app. And I really recommend it. But one level is killing me, and it's a medium-level. I think they're lying. Fortunately, other people have been also been complaining about that particular level. The only one I've seen solve it bought the flamethrower weapon:
But, yeah, if you're looking for a game that will keep you entertained and challenged for months, check out Tower Madness.