As this kind of game goes, this is one of the best I've played. The arrow arch could use some work, perhaps a few more units you can send out, and maybe some spell leveling instead of just buying a new spell all the time could really improve this game.
The only major problem I had with this game was a single glitch. If you have the rock wall down while a dragon is coming at you, and that dragon hits the spot the wall is on, it will vanish and never come back. If the wall actually let the dragons go, or just stopped them without doing anything else, then it would be fine. As it is, one of the most powerful defenses can't be used because of this glitch.
Also, Impossible definitely lives up to its name.
Overall, a credit to the genre, but you lose a point for the obvious glitch.