Origin8 has Sentinel: Mars Defense (iTunes Link) in the iTunes App Store for FREE for a limited time. Deal expires February 21, 2010.

It’s war. Earth’s first off-world colony is fighting for survival against a savage alien onslaught. Mars is under siege and thousands of lives are on the line. Humanity’s only hope is you, the commander of the dropship Sentinel. Welcome to the future of Tower Defense!

Deploy sophisticated turret technology across the Martian landscapeto halt the alien advance. Protect walls and barriers that block the enemy’s path to your base. Upgrade your weaponry, or release robotic drones to repair buildings and harvest resources. Watch the battle for the planet unfold in beautifully rendered, atmospheric graphics.