Anomaly: Warzone Earth is described as a "tower offense" game. It has been a success on many platforms, first on PC and Mac, then for iOS devices, Android, and finally on Xbox Live Arcade last spring. One thing was missing from the package, though: co-op. The Playstation Network release fixes this oversight with the inclusion of a co-op mode for two local players. So how does the "tower offense" work exactly, anyway, and does the co-op make the experience better, or worse?