This was realy good.
i got bored at 16, as it was realy repetitive. just the same 3 enemies. It was Great though. Now, i think it would be better if there where a few more turrets to buy, Either that, or you can upgrade certain Turrets- so when ever you place a Turret, its upgraded to hold more Bullets.
Another Thing would be Auto-Turrets. So you can place them, and they auto-attack. which would be Awesome. you could switch between Auto-Attack and Manual.
Great Game though, i hope you make a sequel!