I'd definitely gather all the designers and artists whose style/vision was similar, then wield/direct them to my bidding, but with plenty of freedom for them.
I'd have the castle over a vast wilderness, maybe hovering over it, (anti-gravity technology is now exposed).
There would have to be a grand observatory tower x10, with a greenhouse at the top full of all sorts of wonderful plants, butterflies etc.
Plenty of windows, windows everywhere, natural light is a necessity. All the stairs would be interchangeably an escalator (made from stone ofc) and/or slide. The slide form having a metal surface, stone may be abit rough.
I like alot of the architecture in Barcelona, so I'd have similar styles running throughout, notably with gothic influences.
Trapdoors and secret passageways are mandatory. Daylight bulbs so no1 gets SAD (Seasonal Affective Disorder). A camera Obscura room.
An assortment of huge horns/pipes like those in Helms Deep, (because the sounds make you feel energised)
Everything would be covered in realistic forest wallpaper, either that or the whole thing could may be made out of glass of varying degrees of thickness, inner walls could contain flowers and more nature. The colour green is essential.
Ok I could go on (if it's going to be hovering, it may aswell be mobile enough to follow the dawn/dusk), there would be labatories for fellow scientists to find out new technologies for making the world a better place, pillow rooms, a room with a huge fan for freefall practice. etc. etc. etc.
All with zero harmful emissions ofc.
Oh shit hangon the library! Everything has oak, oak spiral staircases, bookcases, etc. The entrance is a slide (varnished oak) and as you alight on the floor you can jump on a ladder (that kind that moves along the bookcases) and ride around the whole place because it's all one continuous winding bookcase!
Phew.