It looks like it may need some structural supports to help carry the weight of the overhanging battlements at the top (assuming they're stone). What sort of style were you going for? Historical, fantastic, or as drac suggested, perhaps a cross euro-asian style of castle? Also, is it a military thing, or more a fortified house/manor sort of thing? Those things will affect the shape of the building and what you can get away with. And how "modern" are the internal structures of the castle - medieval castles had really high ceilings (or what did the job of a ceiling) and so the floors (and hence windows) would be separated vertically by a fair distance, whereas a more modern internal structure would have the floors (and hence windows on each floor) closer together (maybe a 10 foot separation), which is what it looks like you have there. The answers to those questions may give a bit more feel for what the building needs (if anything).

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