I love the different level designs. I especially like how spamming towers is NOT rewarded, you have to upgrade smartly to progress.
Some complaints: You can pause indefinitely, and while pausing the bonuses can still recharge. Why even bother with a pause if it lets you cheat and take each wave one at a time, hmm?
The graphics were a bit simplistic too. Not seeing a fisual of machine gun bullets hitting the enemy kinda made it lose a little spark or something.
Finally, it took me about 30 seconds to figure out where I could place a turret, and the walkways for the enemies were confusing as hell. It was all one big mash of color xD
Still, 8/10, good job!