Repetitive, imbalanced.
This game, while it looks good and has a good concept, really doesn't play that well. The game needs more to it, because a horde of 15-20 infantry, upgrade heavily each level, can demolish any castle. Using a variety of unites instead of the swarm tactic makes the game impossible. Also, special units are far too expensive for their power. Why pay fifteen times the money for a troll that only has twice the attack power? I can buy five infantry for a third of the cost, actually have them survive until they get to the wall, and they'll deal more damage when they get there. I'd scrap this concept and use your very strong drawing skills to craft another game.