I'd love to see this expand to a country/world map. That way you could clear different areas. It would also be nice if you kept a certain % of resources from the previous city.
Also, you don't need the bridges. Zombies coming in from random directions would be great at more difficult levels.
A statistics page on the final screen (once the game is won or lost) would be nice. Zombie killed, humans kills, buildings destroyed, structures destroyed, time, etc.