It is very good game. It's not to difficult to beet and that's what I like about it. I like this game a lot. The graphics, the guns, and the music are all perfect.
I advise you to put more things like:
A Bullet Proof Vest (Less damage when shot)
A Machine Turret (Last for one wave and can't buy anymore)
A Rocket Launcher
Attachments for the guns
Heavy Armored Solider
Boss Waves (Best on waves 3, 5, 8, and Last)