good concept, but ...
crappy implementation. the controls are sluggish and unresponsive, making dodging enemies or blocking missiles an exercise in frustration. along these lines, the "recovery time" from swinging your sword is an often-fatal nuisance. also: the sword is utterly, utterly useless after about about the 20th wave or so, if not before then. now, granted that the sword is useless, 90% of your time after that will be spent dodging the enemies and trying desperately to lead them away from the castle while your spells recharge. which, by the way, takes far too long, just like everything else. make the game go twice as fast, and the spells recharge twice as fast again, and you'd have yourself a winner.
now, i love the spell system. love it. but, level ups take too long, bosses give too little XP, and there nothing to do with the last level up or so. once you've gotten all your spells. perhaps you could add in many more levels, with other options such as upgrading the castle, or the sword power, or the player character in some way (speed boosts come to mind).
advice for players: get heal, then holy shield, then the second heal, then lightning, then ball of god. only once you have all those should you start working on the fire tree.