Amazing concept, brilliant execution, solid performance, I could go on about what you did right but some things need improvement. Firstly, an enemy capable of covering ground quickly should be a bit less powerful than the rest. Secondly, the slow moving enemies should deal more damage than the standard enemies in order to punish players who let them catch up. Most importantly would be the issue of ammo boxes spawning where the enemies appear and not giving increased rewards, something harder to get should give better loot.
Despite those issues, like I said, this is a decent game. A few improvements here and there is all you need.