This seems to be a forgotten take on the S.A.S. Zombie Assault/Nazi Zombie formula - unfortunately I can see why it never gained traction. While it seemingly has all of the pieces for a successful game of this style, something about the actual shooting (which spoiler alert is the entire gameplay loop) seems off. In addition, the pacing is all over the place. It ramps up out of nowhere, guns are thrown at you quicker than they probably should be, and the fact that the walls are all destroyable by the "zombies" makes for an experience where you never really feel like your base is in functioning shape.
Everything said, there is still some simple fun to be had.