I’ve recently bought an offer for Sanctum 2 on Steam, and while at the start I was a bit skeptic, I must say that I like this mix between a Tower Defense and an FPS.
Sanctum 2 is the sequel to the world’s first Tower Defense/FPS hybrid game
Pick from four unique character classes and take up the mission of protecting the Cores from hordes of mysterious aliens. In Sanctum 2 you play your character exactly the way you want through the new and extensive customization system. Pick your own loadout of towers, weapons and perks, and pick well, because you are humanity’s last defense against the hordes set out to destroy it.