really good game
Good difficulty. Although, for me, the last level was easier than some of the earlier levels. Maybe it was just the strategy I used. I bought nothing but magic towers and magic tower upgrades, and by the time I reached 5 magic towers doing 8 damage each, the field was a death trap with balls bouncing all over the place.
The strategy doesn't work so well on lower levels, though--mostly due to money and rate of income.
Frost towers didn't seem very useful. They were, but they were too expensive. Fire towers were good, but it'd be nice if they did area damage (or more of it if they do already). I didn't use arrows at all since you kept saying things were resistant to them.
It would help if you made enemies with different weaknesses. Like, make a fire giant that's weak to ice or ice giant that's weak to fire.