I like that the game is different, the playing style differs from normal tower defense. The concept is interesting enough to keep the game going through 12 levels. Some levels work better than others though. "Heavy traffic" was interesting, good gun placements, but a lot of enemies that went after towers, and not one ultimate chokepoint. In several others, the tactic didn't differ too much: have turrets near one point, block the point with squads. Rangers work best for this, as you get the same amount of riot police for less money than with the other guys. Killing the zombies is a task for the towers. I prefer the shotgun tower, as the cannons have trouble hitting anything that moves.
One complaint is that ones you figure this out, the game is pretty easy. The only level I couldn't finish on hard right away is "Gas station", where you can't use the troopers to block the path. The upgrade system works pretty well, but the top upgrades are really expensive compared to how fast you acquire points and how long the game is.
Overall, I like this as a mildly flawed but innovative addition to the genre.