I've been thinking about making a castle cake too and have been contemplating the idea of making the towers out of cupcakes. 3-4 cupcakes per tower stacked on top of each other, carved a bit to get the tower shape. Put a rode through the middle and cover with buttercream from the bottom up. A lot of work and they'd be heavy, but they could be frozen ahead of time and it would all be cake I've only tried this method once, for a very different themed cake, but it stayed...up