I downloaded the Defense-Grid demo and played it for over 2 hours and I really like it. It is very addictive and at only $19.99 it is a great buy. I recommend it highly. Click on the DefenseGrid image to get the demo. You basically put towers on a provided map to slow the traffic of aliens that are on their way to steal your power cores. If they get all of your power cores you lose. The more power cores you save the higher your score.
About the Game
Defense Grid: The Awakening is a unique spin on tower defense gameplay that will appeal to players of all skill levels. A horde of enemies is invading, and it's up to the player to stop them by strategically building fortification towers around their base. Beautiful environments, spectacular effects, and a dynamic, engaging soundtrack bring the world to life. The controls are intuitive and the gameplay is deep — the special attacks and properties of each tower work together to provide many ways to succeed.
* High Replayability. There are approximately 8 hours of gameplay in the main storyline, and many hours of play in challenge modes that give players unique starting conditions and objectives. Each game level is very replayable, and can be solved in many different ways, with increasing rewards for improved efficiency.
* Wide Variety of Enemies. Over the course of the game, players battle 15 different enemy types that become increasingly stronger, and employ a variety of strategies in an attempt to bypass the player's defenses. As the levels progress, the enemies become tougher and more difficult to defeat.
* 20 Unique Levels. Defense Grid: The Awakening has 20 unique environments, each with a different placement of roads, tower build locations, and open areas to plan a strategy around.
* Numerous Tower Options and Upgrades. There are 10 different tower types with 3 levels of capability each. Each tower has unique trade offs that affect ideal placement, such as line-of-sight attack or ballistic trajectory fire. Some towers improve other nearby towers, and some are ideal at specific locations, such as a rear guard tower that unleashes a devastating attack.
* Achievements. Successfully completing each level provides a bronze achievement award. Additional goals provide silver and gold awards for a particular level. A novice player may be able to survive the onslaught, but only an experienced player will be able to get all of the gold achievements.