If you are a fan of tower defense games, here is one to add to your collection. Sentinel, by Origin8 Technology, takes the genre to another world - literally.
In the year 2029, humans have begun to colonize Mars and establish mining operations. Soon alien forces begin to attack and you're beckoned to help. You play as the commander of the Sentinel, a dropship sent to Mars. Organize and build your defense quickly to ward off waves of oncoming enemy troops. At your disposal are weapons in the following categories: Lasers, Bombs, Beam, and Sniper. Weapons have varying degrees of influence but all can be upgraded, granted you have the cash. That's right; all weapons require funding to purchase. Acquire funds for every alien troop defeated. Cash can also purchase repair drones - a must have once you begin to retain damage.