This novel is an excuse for plenty of mediaeval swashbuckling but apart from that it is a mess. Supposedly about voyaging into parallel worlds, it is actually a time-travel story despite some of the characters saying things like, 'of course it isn't time travel. That's impossible.' However, one of the tempunauts leaves a message in the mediaeval French castle he's been sent to and this is discovered in the present day; which sounds like time travel to me.
And if it's a parallel world, how is it that the historical researchers who make up the core of the cast of this story find it sufficiently close to 14th century France for them to base historical research on it? Why should one timestream which has the Middle Ages now be anything like ours in which the Middle Ages were six hundred years ago? There is not enough hand-waving to stop it being a time-travel story pretending to be something else.
And our characters conveniently forget their linguistic issues at some point. They're able to converse in the local language - without benefit of a fish in their ear or otherwise translator microbes - which would not have been English (and 14th century English was, well, Chaucerian.) It wouldn't even have been French as we know it - IMTS are you aware of the congeries of languages that pertains south of the middle of France? They're generally referred to as 'Occitan' but even that isn't just one language, and they have more in common with Catalan than they do with French.
Oh, and then there's the Green Knight. This fella lives in a chapel perilous in the woods and kills anyone who comes near. I really expected him to say "It's just a flesh wound. I'm getting better," as his arms were hacked off, or bounce around on his stumps and yell, "Come back and I'll bite you to death!" None Shall Pass! Yes folks, it's Monty Python time.

- review from February 2001. Yes, just into the present century. And we know how that turned out.