It’s common for fantasy stories to have a medieval backdrop – castles, men (or women) on horseback and fire torches instead of battery operated torches. Of course, writers don’t have to use this setting. They can do whatever they want, because it’s fantasy! That’s the beauty of the genre.
Anyway, by accident I stumbled on a great resource for those of you who do have a medieval type world (like me). The page is called Medieval Demographics Made Easy and I believe it will help anyone get started on a world that is believable.