This game is good, the music & sound design is particularly great and generally it is fun overall. I would say maybe increasing the general pace of the game would be good - I guess it was an intentional design decision to have the player controller be extra slow (to balance going off to find resources vs staying to protect the shield generator) but since you can shoot zombies from off-screen anyway, it feels like the player has quite a bit of freedom regardless
I also might say some general UI stuff like the ability to pause or adjust the volume would be great additions too. Overall though, great job!