Steam now offers Terrorhedron, a 3D co-op tower defense game for Windows. Here's word on the game, which carries a 15% discount for the first week:

Terrorhedron is the 3D, cooperative tower defense game for the PC. Unmatched challenge and complexity is introduced to the genre through 3 dimensional gameplay, micro-strategy through programmable turrets and 'edge of seat' gameplay is extended to multiplayer with up to 8 players online co-op.

Defend wave after wave of colorful Terrorhedron spheres within attractive abstract grid landscapes. Unlock new infrastructure, turrets, upgrades and tracks in your quest to achieve all the stars. Terrorhedron offers a fresh look, feel and an entirely new depth of experience to a popular genre.