Like tower defense games fused with FPS action, then Sanctum 2 should be right up your street as it invades Steam with a launch trailer. Take a look.

Sanctum 2 is the sequel to the world’s first Tower Defense/FPS hybrid game. Pick from four unique character classes and embark on a mission to protect the oxygen-producing Cores from hordes of deadly aliens who are threatened by their existence.

http://www.youtube.com/watch?v=u3f1Pp0_Ijw