The Tower Defense idea is really cool, and it's also a cool game with a lot of potential. The only downside for me is that you don't know which attack you currently have equipped, and how much HP the enemies have left. A shop after each round or at the side of the map would be nice to have. It would have a positive effect on the replayability and general feel of the game if you could upgrade and buy new spells. Adding an Inventory where you can only have a certain number of spells would also add to the feel of the game; you would need to think about spell combinations before each round.