It’s the third week of Microsoft’s Must Have Games promotion. This week’s Xbox Live release, geoDefense, comes from Critical Thought Games. It’s now available in the marketplace.

geoDefense is a traditional tower defense game in which waves of enemies travel along a path on their way to the player’s base. Building and upgrading towers along the way is your only defense. geoDefense’s claim to fame is its Geometry Wars-inspired vector graphics. Everything is made up of colorful neon outlines and explosions are accompanied by a cool background warping effect. geoDefense gets pretty hard eventually, but I found it much easier to learn to play than the existing Xbox Live tower defense games,  Battle for Hoth and Zombie Attack! 2.

geoDefense costs only $2.99 and there is a free trial. Tower defense fans will definitely want to grab it here (Zune link) on the Marketplace.