Needs a bit of work
I have yet to experience this "flying zombies" glitch others are talking about, but i have a few suggestions.
1: make upgrades available for future levels when you already bought them. it gets annoying having to buy the same thing every level to make walls and mines, and by the time i get this done on later levels, my engineer is already DEAD.
2:make the commander's gun more powerful to start with. i know there are upgrades for it, but this just points back to my first suggestion.
Overall, a good game- it just needs those things fixed, and I'm sure you might get the front page.