Like with Age of War, you can only win at the final level, and it's pretty easy to get there and max everything out. Money doesn't matter in the endgame, so you just spam the most powerful units and wait for your level 5 summon to charge. This means all the other stuff you put in the game, all the units and turrets and abilities, they don't matter. There's no strategy involved in the sense of weaker units being preferable because you need to save money, or needing to buy more turrets because your old ones can get destroyed or something. It's like most of the work you did on the game is just wasted because of lack of more involved game design.
Not only is the fine-tuning way off, but I think there may be an actual bug. The difficulty levels don't seem to make any difference. Easy and hard feel the same.
Also I want to mention that this is not tower defense. It's a one-dimensional RTS, like Age of War