One of the best games of this kind
Concept - very very good.
Art - very nice
Gameplay - no bugs, no errors, smooth
Difficulty - easy/moderate. Once u get the mechanics of the game, it's not that difficult to get past the levels and survival mode.
I demand a sequel though! :)) I really want to see more games of this kind.
If possible, try to add a wall upgrade, maybe some turrets (the snipers are very weak) and some flying units.
Also, more levels. I found out the AI weakness fast and from that point on, the game was really easy and short.
In conclusion, GREAT JOB!!! You can do even better next time.