Neat idea
It's a neat game, and kind of fun to play, but I do have two problems with it. I can forgive the music and the graphics, but the first one is not having a menu screen. It can get kind of annoying having to constantly reread the instructions.
Secondly, there's no flow to the way the baddies come. I'm guessing the odds of a baddie coming is completely random. You need to make less baddies come on the first castle and they have a steady increase for each additional castle. Mayhaps keep it random, but have it have low odds... (say a 90+ on a random 1-100) and have each castle add an extra ten to the random number generated... If the last sentance made no sense, disregard it...
But it's a fun game, and has a lot of promise.