Turret time!
I love strategy and defense type games and this was no exception. It has a lot of replay factor, good difficulty, and interactivity.
However, I didn't understand some of the game mechanics. The second level was especially difficult, and the main reason was lack of "resources." You can have up to 5 turrets in the beginning, and rebuild one after it's destroyed. But once more are destroyed, it refuses to let you build any more (until you are 1/2 second away from losing). Money wasn't the issue, and neither was the power level--you simply could not build more turrets.
If it's a numbers display issue, I hope it gets fixed. Otherwise, this was really enjoyable to play. Nice job.