That easy vanilla cake is my cake of choice too. Have made a castle cake similar to smartiepants using Swiss rolls and ice cream cones then just covered it all with buttercream, decorated with dolly mixtures and smarties.
I think I used Nigella's chocolate brownie for the bottom! My theory there was that the party guests would eat the towers and I didn't want to have an enormous cake that wouldn't get eaten. It's such a big cake overall. The children never eat the cake at a party. They just lick the icing off.
I've made this twice and the second time I used bought Swiss rolls for the towers which were much more successful. I also used ice cream cones and I shoved wooden skewers through for flag poles and put the sweets off one of those bracelets on them.
Check out the picture on my profile, I made a smaller L shaped one and put it on top, covered it all in icing and made a small castle out of cake that I covered in buttercream, isce cream cones for turrets. I bought the Disney Princess figures off of Ebay. I was really pleased with it!
You could just make square cakes and pile them on top of each other to get the height you want, cover it in pink buttercream.