Epic.
One of the best webgames I've ever had the pleasure of becoming addicted to. Simple controls, diverse (and difficult) enemy units, and a 360 degree defense perspective with easy-to-execute commands, not to mention a little upgrade system to boot. The simple graphics actually made this game kind of nostalgic and easy (and fun) to pick up. The music also kicked ass. I can't find any real flaws that detracted from the gameplay.
My friend, you have here a masterpiece. I hope that you make more of them. Great job.