If I ever become obscenely wealthy, I wouldn’t build a castle.
Well, I would, but it wouldn’t be a military fortification or an ode to my own wealth.
My castle would be a humongous gorgeous library, with extensive grounds for outdoor reading in a variety of environments (or maybe biodome-greenhouses built in, like the Butterfly Conservatory), and a few huge aquariums with clear tunnels leading to underwater dome-room reading nooks, and huge rooms with fancy ceilings and spiral staircases and columns in various styles.
There would also be a computer room, and a cozy movie room with a nice couch or a few armchairs and one wall that’s the movie screen, but the books would be the main focus.
The windows would be those bay windows, adapted into reading nooks. There would be reading nooks all over the place. Furniture style would vary - there would be squashy couches and stately armchairs and beanbag chairs and wooden benches and rolling spinning office chairs, possibly grouped according to style/theme, but possibly all mixed up.
In addition to various fancy dramatic staircases, there would be sweeping ramps between floors, for times when I just don’t want to take the stairs. Also an elevator, perhaps with designs on the doors reminiscent of those rattly old cage elevators - I like the look of wrought iron.
There wouldn’t be freestanding shelves in the middle of most rooms. The bookshelves would be built right into the walls. Every or nearly every wall would be a bookshelf.
And when I died, it would be in my will that the castle becomes a public library (which is the other reason for the ramps, so it’s all wheelchair accessible) and my wealth would go towards maintaining the estate, and paying the staff who ran it.
(Also there would be lots of ‘single-stall’, fully accessible, gender-neutral bathrooms, with showers built in and personal grooming products available on request.)