It's very good, I would say that the strength of this game is in the environmental theme and it's history. The gameplay i liked the purpose, change characters for different combat modes, its interesting.
What I can say about the gameplay, making the actions "quick" and the little feedback that a horde is about to end, the life of the zombies, etc...
About the investigation or exploration of an area, not bad actually, I had some bugs with some colliders, but i liked the idea of being able to search for something. Good Job!