I've seen this concept before, and even thought of recreating it myself as a project earlier this year, but settled for a normal TD ;(
your take on it was really nice and strategic, I know first hand how hard it is to balace these TD games, especially if you're playing in reverse, and you did a great job doing so! :)
Also, I really liked the artstyle and music of your game, it added alot to the game's charm ;)