Good idea, poor execution
The last section of the game where you must build the castle is virtually impossible to beat with the goombas dropping down on you, and you have nowhere safe to create the tower. You have to constantly switch back and forth to keep alive, and while you're doing that, you can't kill the goombas coming down to attack you. This is a major flaw as far as I see it. It goes past the "challenging" aspect and into the "play this game for an hour before you master it just to get passed this one section."
Overall, the game has promise, but unless you think stuff through like that, it isn't going to fly with most casual gamers methinks.