Nice Game
Easy to play, keep your interest, not too easy, too hard. Could use a little work though.
-Money becomes useless once you've bought all the guns. I had 80k and nothing to spend it on when I finished the game. This could be solved my adiing in:
-Could use more upgrades. I like the system you have now, but it could do with soem tweeking. Maybe add in an RPG element to give you exp based on the kill, color, distance, and wave number to give you levels that give the skill points. But also, you could add in a few more upgrades like reload speed, rate of fire, and accuracy that cost money to upgrade.
-More guns maybe? A little more variety couldn't hurt. Although, I just used the USP until after wave 2 when I bought the M21. Never needed to use any other gun after that.
Keep up the goos work, looking forward to Duty Hill 2.