I'm building a Royal Estate - (merged with "what kind of rooms are in yours?")

This sounds awesome! I would probably add a dedicated music room somewhere. Also a conservatory which is like an indoor greenhouse (like the one at Biltmore). Biltmore has some interesting basement areas too such as an indoor swimming pool, a bowling alley, a gym, and a dedicated Halloween room. I would look up some other grand houses like that for ideas.