well.. as the old proverb goes - assume makes an ass out of u me ... you have to find out what its supposed to do as the buildings have to live up to different codes depending on their usage. For instance, should tourists be able to stay there (sleep & eat) which would set requirements for the number of guest chambers and toilet facilities and also kitchen and quarters for staff and such.
Also think about the placement of the building - there are rules for how much of a land plot a building may take up, and probably also how a building may look so dispensations might be necessary. I addition to that some landscaping would probably be nice as well - to really set of a Castle a some what mountainess region is best - but generally the more exiting the landscape around the castle the better imho.