Good, but a little hard.
I found the first two levels quite easy, and the graphics are impressive. However, after that, it seems to increase in difficulty too quickly for the level you're at. I really liked the idea of the heroes though, and perhaps something to think about in the future is the ability to customize your hero's appearence slighty. I do think you've tapped into a well needed addition to the simple TD games that seem to be 'in' at the moment.