This is the third book you've written that is set in the Middle Ages. What do you find so interesting about that era?
The Medieval Period was a watershed time. The Age of Discovery was opening the Americas, trade was more global than ever before, the monarchy was giving way to mercantilism which grew into the middle class.
I personally don't care about the lives of royalty: their stories are overtold. Tradespeople and craft workers make interesting characters because we recognize the products they make, but the processes are very surprising and so, I hope, interesting to readers. The characters are enough like us for readers to identify with them but enough unlike us to surprise the reader as well.
There are also advantages as an author to writing about the Middle Ages: communication was very slow which makes it easier to work with characters who don't have access to information or each other at the speed with which we communicate. It took a long time to travel during that period; there was almost no postal system. It's a time that is less primitive than the early post-Roman era but less well trodden than the Elizabethan era.
What are some of the disadvantages of writing historical fiction?
Historical fiction based in the Middle Ages is going to be inaccurate on a number of levels. While women certainly had power and agency, the majority of them did not and it gets very boring writing about powerless women for the sake of accuracy. Having more than one comment from a man about the oddity of a powerful woman is just jarring to the story. The Medieval Period was also a violent time which is unpleasant to read about, as well as a time when people were very religious which I have no interest in. That means that you are writing about independent, non-religious women who are able to escape much of the endemic violence. You strive for accuracy in so many aspects of the work, but that core conceit is necessary.