The plants could go for some shading. I think they are a little too bright. Besides that, the polygon shading is pretty good. The lower tunnel could slightly use more, but it isn't a necessity.
For it being a Zombie map, the layout serves a challenge. It honestly would be a good one. If you decide to make it into a Zombie map, I suggest you make two versions of it, one being as it is and the other being the Zombie version.