Really, really good game. The idea is great and the controls really simple and intuitive. The artwork suits the genre perfectly and the music is pretty good. The AI is also pretty good; it can be quite a challenge.
One thing I would suggest though, is to make it easier to adjust the percentage of units. I would probably make it so that it was not mouse controlled, perhaps with arrow keys. It's just that when you're trying to co-ordinate attacks, it becomes much more difficult when you have to keep moving your mouse back to the bottom centre.
This is a very enjoyable game. The simplicity and solid execution in all of its facets really lend to a polished, innovative and entertaining game.