Sanctum 2

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.

Sanctum 2 is not just a simple upgrade of the Steam hit that was Sanctum 1, it’s a new game completely rebuilt from the ground up using community feedback and outside-of-the-box indie game design to really push the bar of what an indie-studio can do

Source: The Developer Website.