There are many castles, palaces and aristocratic houses where you can rent a room or an appartement.
Sometimes the vacation rental is in the castle garden, where you have a lot of privay but still can enjoy the charmes of castle live. Often you will find that your host is the castellan himself, since a lot of these houses are family-owned estates. So you will sometimes find your breakfast is served by The Count & Countess!
Here are some recommendations for Castle bed and breakfast per country:
France - Belgium - The Netherlands - Germany - Austria