Okay I was thinking about this for a little, but maybe there is a reason of why Minnie stays at the castle. And yeah you could just say ‘someone needs to rule over it’ but I don’t think that’s it, if you look Minnie uses some strong light magic.
^ Like this. This is pretty fucking strong and the lock on the door is pretty strong as well. So what if she can’t leave because of the Cornerstone of Light? What if all of her strong light magic comes from it? So if her magic comes from the Cornerstone of Light wouldn’t it make sense for her to stay close to it and protect it? And don’t pull up the fact of Disney town, because I believe Disney town and the castle are on the same world. The name just changed between Kingdom hearts 2 and BBS.