These bundt type cakes are usually just served plain or dusted with pwd sugar. Here some that I found:
http://www.coolest-birthday-cakes.com/cake-picture.html
http://jillystakesthecake.blogspot.com/2008/03/princess-castle-cake.html
You can cut out fondant block for the stones and attach them.
Maybe someone else can give you better ideas 
Could you try a thin icing or a pourable fondant and pour it over the castle. That should keep the definition. Then you could decorate as you wish.
Maureen
Could you try a thin icing or a pourable fondant and pour it over the castle. That should keep the definition. Then you could decorate as you wish.
Maureen
I'll let you know how it goes! Another idea is to frost it, and it doesnt matter how smooth or defined it is if you maybe cut little fondant squares like tiles. Then cut some in the shape of the doors and windows and stick it on. The pourable icing idea is good too.
I tried the poured fondant and didn't get the look I wanted. The lady I was making it for said she wanted buttercream so I tried to pipe it all in. Unfortunately I sliced my finger on a glass and was having trouble with maintaining pressure on the bag. I'm disappointed because I think it looks sloppy. I delivered this morning though and the customer loved it!
I tried the poured fondant and didn't get the look I wanted. The lady I was making it for said she wanted buttercream so I tried to pipe it all in. Unfortunately I sliced my finger on a glass and was having trouble with maintaining pressure on the bag. I'm disappointed because I think it looks sloppy. I delivered this morning though and the customer loved it!
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