At last year’s C4 Literary Conference, I learned a lot about building worlds from Kelley Armstrong, author of the Women of the Underworld series. (One of the books, Bitten, will début as a television show for Space.) Although set in ‘our world’, she still had to set some rules for her supernatural protagonists. Her werewolves need society rules to keep from getting discovered, and she had to figure out what rules would govern her lone vampire character, and that’s only two I can remember off the top of my head from the session.
In order for reader’s to suspend disbelief, a world needs enough structure to make it ‘real’, and I think that what made a place like Hogwarts real to Harry Potter readers. J.K. Rowling had notes about Diagon Alley with a sense of its location in Muggle London.
Have a look at a short video on world creation from author Kate Messner from TED-Ed: