This shouldn't have surprised me, but there actually ended up being a fair bit of hard work mixed in with the fun. Because, as any outliner knows, jotting down some notes on how things are supposed to be is a far cry from writing the thing itself. I had to keep stopping myself and going, wait, if it works this way, but this other thing is also supposed to happen... Hmmmm.
Because of writing this little bit of backstory, I have clarified a whole whack of world-building, sketched out a layout of the main palace and its grounds, and made many notes on things I have to check when I'm revising the novel itself.
It was still fun, of course. And I highly recommend it as a technique for anyone who wants to make sure their characters' world works. Pick an important piece of backstory, and flesh it out. You might be surprised how many things come up that never occurred to you before!