Sanctum 2

Sanctum 2 is a first-person shooter game, the second installment in Sanctum series by Coffee Stain Studios.
The story takes place on planet Loek-III colonized by people. The colonies are built around the Cores, large spheres of energy that make Loek atmosphere suitable for humans. You take a role of one of four Core Guardians team members. There are 4 characters in the game with its unique abilities and characteristics and several unlockable weapons for every one of them.
The gameplay may be essentially described as a mix of FPS and tower defense. Every level is necessarily a tower-defense-style map with the main building, the Core, which player has to protect from waves of enemies, coming from the access shafts. Every round consists of two phases: building phase, when the player can build towers and walls that help hold back waves of enemies, and defending phase when the player has to kill enemies in order not to let them destroy the Core.