Well, I did make one change to the era that's actually a lot more significant than Geoffrey's date of death or John's ability with a claymore. And that's attitude.
Or more precisely, attitudes.
I gave all the characters in TWS modern attitudes and personalities. In the real Middle Ages, Henry would never have left his monastery (why did he do it? how? a story for another time). Medieval attitudes toward history, community, individuality, science, faith, and bigotry were profoundly different from our own ("our" meaning fantasy readers in the Industrialized West). But Henry, Alfie, Mattie, even Geoffrey are all pretty damned modern. I'll cop to it, because I was trying to have some fun, not be completely accurate.
In fact, I'd argue that those novels that *do* strive for for historical accuracy, like Follet's excellent Pillars of the Earth, are dishonest in their own way...because we'll never truly know how medieval peasants, masons, architects, or merchants felt - but books like PotE give the illusion that we can. So there.
(But you should read PotE and Mary Stewarts The Hollow Hills if you haven't already. After you've read The Wrong Sword, of course.)