I liked it... But...
I liked it. It was fun, for a while. However, it did get rather dull just shooting with the same terrain, same background, nothing new.
One way to impliment upgrades would be implimenting civilians who you shouldn't shoot, and when they pass you, you get some funding; aka cash, which could be spent after every wave or so. Upgrades could be increased numbers of air strikes, better weapons, grenades, mines, perhaps even have the target to build the wall with the funds you get, without letting any of the zombies pass. You could also get a sniper tower or something else to back you up.
In all, a game with much that could be expanded. But it was still fun.