Cool game
I like what I saw for the most part and thought it had potential.
One thing that was annoying the hell out of me was that the UI elements at the bottom of the screen kept getting in my way when I had to fire at something in the lower portion of the game play area. Consider putting the UI in a border region and leaving the game play portion blank so you can target everything without having your vision obscured.
Also, I wanted to click or hit escape to remove the information windows that appeared once a wave had been destroyed.
Well done.