Not bad at All
Concept and idea are great...the problem is implementation.
I spammed Dragons the whole game and won..used wind a few times to dispel some of the opponents magic - that was it.
I suggest:
A. More Levels.
B. an upgrade system and\or abilities being gained as the game progresses
Doing so will give the game a more varied learning curve (as you'll have to use certain spells and elementals as the game progresses) and allow some variation in game skill hardness.
All in all this is a good and solid concept, fun too which could use some improvements for a good hard to earth defender type game.
Hope to see an improved version with the sequel !