Before Defense Grid, there had only been one previous tower defense game on the Xbox Live Arcade. The game is ported from the PC, however there are no signs that this was the case. Plus at only $10 it's only half the cost and includes more maps. For those who dont know the genre, tower defense games challenge you to think ahead and plan what towers to place, and where to place them. You only have a certain amount of spaces to build them, but the towers are upgradable. Of course, the towers are good against different enemies, there are towers that work better firing at groups of aliens such as the Inferno Tower. There is a quirky British AI who lost the fight to the aliens over 1000 years ago, but he left part of him behind in the AI, and serves as your guide.