a diamond castle cake. not flat, oh no - because i did a normal castle cake for her friend. she wants a proper 3d diamond castle cake and is so excited. have you seen the diamond castle? its see-through! help me, help me...
How about doing the floors, doors, middle etc out of cake, but then cloaking it in something glass-looking? Maybe rice-paper (wouldn't look as solid as icing)...um...am sure someone with better ideas will be along in a mo - good luck though!
Don't worry about the see-through aspect -- do it in white with lots of glitter/lustre powder.
If having to make one, I'd be inclined to do the central part of the castle as tiered cakes with a longer thinner cylinder on top, and then do all the turrets and parapets and so forth in sugarpaste or pastillage, depending on fiddliness.
Debbie Brown's MagicalCakes and EnchantedCakes books might give you some ideas that you could adapt.