Would you be able to post a picture of the finished cake? I am curious only because i once made a castle cake for a four year old friend using store mix. I used a lot of different candies for various decorations, gingerbread for the walls, crushed oreos for the "dirt" surrounding the blue Jello moat and gumdrops for the driveway, and a Hershey bar for the drawbridge, all on a piece of aluminum foil wrapped plywood.
Not very professional looking but it impressed the hell out of the four year old and his little friends. I've been curious about improving it ever since and considered using things like sugar cubes and royal icing to build the walls out of.
Anyway, I'd love to see what you end up with, whatever cake you decide to use.