Well-balanced, with a nice touch of history.
I love this game primarily because it's tower defense game that goes where almost no tower defense game has gone before: the European conflicts of the 18th Century. That single fact alone earns you an 8/10. Gameplay is also smooth and challenging at the same time, not to mention that achievements actually DO SOMETHING for you rather than just being in a showcase that no one else will ever look at. Now that's innovation, which earns you two more points.
Overall, very good game. Keep up the good work.