Really good concept.
With a few changes I think this could be awsome. By round ten I had everything I wanted and didn't need any more upgrades, so possibly some more expensive powerful options would be a good idea.
At the survival round all I did was spam confusion spells, combined with cheap swordsmen until I could buy a Commander. I might have lasted longer if there were more slots for your own choice of soldiers or spells, instead of Zap, which became pretty pointless when the enemy sent in those Battle Healer guys.
The Battle Healer guys themselves were pretty annoying too, as the constantly went around healing each other, and everything in like a 4 mile radius, and Zap couldn't take them out in one hit so they just healed again.
And finally, I'm not really sure why it kept happening but occasionally my units would burst into flames and run away into the trees. There was no warning it was going to happen and I couldn't manouver my guys away from whatever was causing it. I lost a good few Commanders that was.
Overall though, I really enjoyed it, and I played it all the way through to the end.