its really good
im gonna say to "wearfare" few comments below me. im not sure how many tower defense you played or there are where the monsters attack the towers. Secondly, it doesnt even sound like you even played a quarter through the whole game. Thirdly, sending in just masses of huge numbers of units WILL get extremely boring after a short period of time as waiting for huge amounts of units to finally get to the end wastes awful lot of time, this game maintains entertainment by having small amounts of units that needs to be placed in certain orders while some being used more or less in certain maps.
The spells gives a nice bonus to the funness of the game.
I found it funny how your upgrades make your castle all big and mighty looking and all suddenly you just add a rainbow and circus to it lol. one thing you should do to this game is add another speed ++.