And for that your greatest problem is the Reader Who Knows Everything! Really!
Some of them are merely unfamiliar with the past. Like not knowing that "Your Majesty" is modern in reference to kings; a medieval king is "Your Grace," and a medieval "Your Majesty" is an emperor. Or assuming that teenaged medieval princesses obviously ought to be running around as freely as teenaged medieval princes and objecting that a princess is being kept carefully watched. . . .
Then there are those who refuse, no matter how carefully you inclue them, to believe that your imaginary world is not a figment of your imagination and therefore it can be wrong if it does not include the exact details of the history you are ripping off. Like feudal duties that don't exactly match those of medieval Europe.
Or perhaps, those stretches of medieval Europe that he was familiar with. That did cover a large chunk of land for a thousand years. It might, perhaps, be wise to assume that some variation was inherent. On the other hand, this shades insensibly into the person who Knows, absolutely, what was happening and therefore your story could not have happened. Wives were always executed for adultery in the Middle Ages, and therefore a story about one being repudiated is wrong even though in fact many were indeed repudiated, and I was ripping off a real world incident. . . .
How much to inclue is a judgment call. There are people who still won't get what is going on even when you have put in enough clues that a reader who picks up on one or two clues will be seriously annoyed at the way the story hammers him over the head. But that's trade-offs between readers of good will. Those who aren't willing are annoying but fortunately easy to classify into "not part of my target audience."