Beat it on hard mode. Some levels were easy, while a few required a ton of trial and error and were unnecessarily difficult. And even then, sometimes once you got to a certain point with your towers the level became too easy. Overall a very bumpy difficulty curve, even in individual levels.
Towers are poorly balanced. I used the secondary upgrade for Earth, Air and Water, and I didn't even touch the first upgrades or even the Fire tower. Fire was WAY too expensive and I got all the utility I could ever want out of the other towers.
My strategy was the same on every level: put multi-rock towers near the entrance, and litter the rest of the area with AoE freeze towers and Air push-back towers. The freeze + push back is a completely overpowered combo that trivialized a lot of the larger monsters, but at the same time I feel like the game would be impossible without this tactic. One freeze and 2 pushback towers basically made bosses sit still, especially the last boss who traveled ridiculously slowly for "is strongest enemy".
I did really like the monk at the end, though. I like the idea of ALWAYS having a back-line of defense, even at the beginning of the game.
This game has some major balance issues and was more frustrating at times than I'd care to admit, but damnit I played all the way through on hard in spite of it, so congratulations on a job well done.