Many of the rooms in the castle seem to exist for no reason except to be beautiful or strange or wondrous.
There's a ballroom where empty suits and empty dresses dance together. Invisible hands are set on silk waists and shoes move across the marble floor, attached by nothing but air and the knowledge of where legs would be.
There's a hall with mosaics made of broken glass, designed to look like you are under the sea. Brightly colored fish dart by, changing the color of the fragments as they go. Occasionally, there is a shadow across the glass as something large passes by, far above in the nonexistent sea.
There's a room full of paper songbirds. When they sing, you can watch the musical notes drift out of their mouths. Other birds swoop by, snatching the notes from the air and then singing them out again, mixed into new patterns. Occasionally a bird will run out of notes and crumple. Occasionally a bird will gather so many notes that it can no longer hold them and will lay a paper egg.
But there is practicality mixed in with the extravagance.
The left side of the castle contains the guest wing. Each room is unique. A few are meant for children.
There's a room with a tree containing a cradle. It sways softly and constantly in a nonexistent breeze. There's a flower growing out of the wall next to the door. A caretaker can tuck it behind their ear, and hear everything that happens in the room while they're away.
There's a room with a crib made of a sea shell. It is coated in mother of pearl and big enough to hold a child. In this room, the sea can always be heard. A sandcastle builds and falls and rebuilds itself in the corner. The sand always stays nicely where it should be. There's a small seashell by the door here and one needs only to hold it to their ear to hear their child.
There's a room where the cradle is a cloud. The ceiling is painted like the night sky. It matches the true sky perfectly, except for the unusual frequency of shooting stars. A small cloud will follow the caretaker when they leave. It unobtrusive, hiding in corners or beneath chairs, unless there is trouble.
There's also a room with a simple wooden crib. It seems to be a concession and not one made happily. Everything in the room is almost purposefully dull. There is a mobile above the bed. From it, hangs simple circles painted with dull colors, blue, red, green and something that might be yellow, if yellow was sad. There's a plain wooden bracelet by the door. It reports the sounds in the room completely accurately. The volume is not adjustable.