Defense Grid is a neat take on the tower defense genre, created by the lead designer of Age of Empires II, and released for both PC and Xbox 360 (interestingly, it's at a discount there as well, at 400 points), although not simultaneously.

Basically, you have to aid an AI called the Entity in defending its homeworld from an alien invasion. To do so, you have to protect energy cores in a sucession of maps by strategically placing towers with weapons that range from flamethrowers to lasers to electric shocks. Aliens enter the maps in waves through a predetermined path which you have to cover with enough firepower to prevent your cores from being stolen.

The interesting part comes when you realize different towers are useful for different enemies, that you can't place towers anywhere you want, except on a few places in the map, that tower effectiveness will be affected by position, line of sight and other things, and that eventually, the waves of enemies will overlap as new ones will keep coming while you're still busy trying to wipe out the previous ones, which leads to much dismay and clenching of buttocks. Each enemy destroyed nets you resources to either build or upgrade your towers.

It's a pretty good game for a budget title, and the dialogue from your AI companion is all kinds of awesome. So if you are into the genre (and even if you're not so much), this one might be worth a shot.