Sanctum is a pretty neat UDK-powered game that's a mix of co-op FPS and Tower-Defense and it is available on Steam. The free "Elysion One Overrun!" update is now out and includes a new map, Shotgun, and two new towers.
A few new pieces of paid DLC are out as well: two new maps: Aftermath and Aftershock add two new towers: The Violator, Penetrator, and Killing Floor. All of these are available for temporarily at 10% off.
A few new pieces of paid DLC are out as well: two new maps: Aftermath and Aftershock add two new towers: The Violator, Penetrator, and Killing Floor. All of these are available for temporarily at 10% off.