Really good game!
The originality behind this simple concept - reverting the Tower Defense idea - goes beyond any word I can type here. You have created a simple, yet extremely fun and adictive game.
The small details are what I liked the most: you are not just clicking "Upgrade" buttons, it has an in-game art effect on your Castle. More than that, your achievements are also reflected in-game. These little details are signs that you have really dedicated to this piece. Congratulations, man.