Great game with nice design elements! However, game is too easy. The only difficulty really comes from Ninjas. So my suggestions 1) Enemies in general need to be tougher other than the ninja 2) fear effect needs to be weaker 3) there is very little synergy between the towers 4) Spell hotkeys would be nice.
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Canabalt"){kind=icon}
 "Valthirian Arc"){kind=icon}