Neat game overall! I thought the idea of protecting little areas and upgrading them so that the zombies couldn't get to them. The achievements were a very nice touch, and the ending little cutscene thing was cool.
The only things I thought that could've been better was that it was very hard to explore without the cars from being destroyed by zombies. Showing the current health of the cars would be helpful. Also, at the beginning of each run, I had to rush to the axe and back, and usually the car was almost destroyed by the time I returned. Maybe making the cars being destroyed not end the run, or increasing the player movement could help.
Overall, I thought you did a really nice job, and I liked some of the small touches that you included. The ending was satisfying to see.