Here’s a medieval history related thought for all those interested in the period, writing historical fiction, or writing the kind of fantasy where everything has a medieval feel. It’s about countries, and is that the notion of countries could get quite complicated sometimes.
First, the notion of a defined nation state with fixed borders existing separately from a particular collection of people wasn’t consistently in place. Philip I for example was very definitely a king of “the French” rather than of “France”. Indeed, separate peoples within a kingdom would sometimes have their own set of laws that applied to them regardless of where in the kingdom they were. So a Norman might not be subject to the same rules as a Burgundian.
Secondly, countries were very much developing in Western Europe. By which I mean that the places we think of as France or England only came into being during those periods. England as we know it today could be said to have finally been formed in 1054, with the completion of the project of bringing together smaller kingdoms like Mercia and Northumbria (literally the region north of the Humber) and the expulsion of the Vikings from York.
Then there’s the question of royal control. For much of the period it came down to personal power and influence. Most kings in the early part of the period had to essentially re-conquer the kingdoms left to them, at the very least processing around them, but often having to fight and depose lords. Many lords could end up more powerful than their kings, staying as subordinates in name only, as in France, where Louis VI began his reign as essentially master of only the region around Paris. This was the period in which administrative government only just began to extend (with kings like Henry I and Philip II creating whole classes of ‘civil servants’ and the accompanying paperwork)
Finally, remember that borders were fluid. Much of the North of England ended up owned, invaded or raided by Scots at one point or another, while at the same time, the holdings of Anglo-Norman monarchs extended well beyond England. It was a time of shifting changes, not fixed ideas of nations.
All of which means that you should be quite wary of large nation states in your fantasy. The idea of large, quite clearly ordered countries with strong collective identities and traits is common in fantasy, but it’s really not that accurate. It also might not be the most interesting way to do things, since the chaotic historical reality offers so much more scope for interesting stories.