Honestly, I think this is overly complicated for a tower defense. Though some of the ideas are interesting, overall, I feel it just dilutes from the fun of it. I don't mind the idea of elemental strengths and weaknesses, and even skill trees on towers in games is pretty sweet... But only having 2 towers per hero is really lame... Not to mention there are so many flying creatures, that getting Mellie is simply overrated.
For the love of god, add a slow aura or invis detect... Getting a brilliant on level 11 is fricken impossible... The last boss gets to your gems while invis
 "Arkandian Crusade"){kind=icon}
 "Demons Took My Daughter"){kind=icon}
 "Protector IV"){kind=icon}
 "Epic Battle Fantasy 2"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Viricide"){kind=icon}