A fine example...
of how not to do a defense game. This was *so* bad, I got an account on Newgrounds just to rate it.
One: You need a variety of enemies. Not just the same enemies used time after time, with bonuses to their speed and health.
Two: Wave length was *way* too long. You don't want your players wondering 'dear god, when will it end?' halfway through a combat wave.
Three: Too few upgrade options for the turret, and most of them useless. Why would anyone buy health, for instance?
Four: Way, way too easy. I got through 7 levels before quitting in boredom.
If you want an example of how to make a *good* shooter, look up All Hallow's Eve. That has a variety of enemies and weapons, and the various upgrades you can buy actually appear on the screen.