Good but not quite balanced
Ninjas, for one thing, were very difficult to deal with effectively enough before they run away with my gems or at least touch them, messing my chance to get the best rewards...
But mostly, the different ranges of towers let the crypts (having the longest range) gain levels very quickly, killing enemies more efficiently and "starving" other towers, in particular temples. Perhaps towers should have different XP requirements for leveling up?
I did like the simplicity, surprisingly - three spells, three tower types each of which has two possible paths later on. Better than having too many choices to develop a strategy which I can remember :) And the spells were quite necessary at times. Upgrades were nice, though some more effective than others. Very good game overall, I'd go for Brilliant on every map if it weren't for the ninjas...