Sanctum

Sanctum is a first-person shooter tower defense video game. The gameplay in Sanctum can be divided into the building phase and the extermination phase. In the building phase, the player builds structures on a pre-defined grid, using the 8 'towers' chosen at the start of the level. These towers include stationary weapons, teleporters for the player's use, and pads that affect enemies moving over them. The player can also upgrade any of the 3 weapons equipped at the start. (wikipedia.org)