Tap Defense
Tap Defense pits you against monsters from the underworld who are trying to break down different barriers just to get to heaven. Offered absolutely free in the App Store, the game is considered one of the best tower defense strategy games available for free.
It was created by TapJoy and since it's release it has become of the biggest, most downloaded free apps in the iTunes library.
Of course, it's not just because it's offered for free that makes it great but it's actually a very addictive and entertaining strategy game. Combining strategy and good old fun, you'll certainly enjoy this one. Why not go ahead and check it out?