Com2uS Inc. has Tower Defense: Lost Earth HD (iTunes Link) for the iPad in the iTunes App Store for 99-cents for a limited time.

Tower Defense: Lost Earth takes classic defense games to a whole new world.

In search of resources that have been exhausted on Earth, you followed the probe across the galaxy searching for habitable planets. Take command of towers and special weapons to hold your defense against endless waves of enemies on the hostile alien planet.