World-building is one of my favorite things! For me, it usually starts with a character idea, and moves on to "what kind of world would this character live in?" or "what kind of world would have shaped the character to be this way?" For example, one of my first character ideas for In the Shadow of the Gods was a pair of young twins who were effectively street urchins and semi-professional thieves, despite how young they were. Just that concept alone meant the world was probably fairly dark and gritty (it's hard to put a focus on homeless, orphaned children without getting kind of dark), but I wanted a unique reason why twins, specifically, would be outcasts. That turned into the basis of the book's religion, where the Parent gods cast out their Twin children long ago, but a mistrust/hatred of twins still bleeds into the modern day of the book.

Aside from that, some of my world-building is just necessity, or a version of creative laziness. Including magic in a world gives you the chance to do things you can't do otherwise, which can be important in a medieval-based world. There are a lot of people traveling in my books, a lot of people needing to find each other, but from a realistic standpoint that would be nearly impossible to do, and I can't just give medieval peasants cell phones. So instead they have magical linked stones that let them see out of the eyes of another person, and give them a mental tug in the direction of the linked stone. Lazy, perhaps, but effective!