Have you read Roger Zelazny's Amber novels? Ten novels (also available in one giant book containing all 10) and they're kind of a mix of medieval and modern fantasy, with a touch more of the medieval in my opinion. They're absolutely fantastic.
I also love a lot of his other novels, though they're not medieval fantasy.
- "A Night in the Lonesome October" may just be my favorite fiction novel, I read it at least every October. I'm not sure how to classify it. It features a lot of horror characters (the main character is Jack the Ripper's dog) and it centers around opening or sealing shut a gateway for the old gods, but despite that it's not a horror tale, it's more of a comedy.
- "Donnerjack" (with Jane Lindskold) is modern fantasy, but it's really good. Cyberspace somehow links with myth and merges. It's got one of the best uses of Death as a character, rivaling the Death of Discworld, IMO.
- "Bring Me the Head of Prince Charming", "A Farce to be Reckoned With" and "If at Faust You Don't Succeed" (all with Robert Sheckley). These three are comedies about the contest between good and evil to decide who will rule humanity. They're absolutely hilarious. I think the setting may be Renaissance, but I can't remember for certain.
- "Forever After" (multiple authors) This is a comedy about what happens after a fantasy adventure ends. All the mystical items brought together to defeat the evil overlord are now wreaking havoc and have to be hidden away. Each of the adventurer's tales is by a different author. Hmm... come to think of it, this one is medieval fantasy.
- and a lot more. "Wizard World", "Damnation Alley", "Doorways in the Sand" and others.
I also want to reccomend "Changer" by Jane Lindskold. I could try and fit it into your criteria by saying that it does feature King Arthur as one of the characters, but it's more modern fantasy, really. The characters are all old school (Enkidu, Anasasi, Gilgamesh, Loki, sasquatch, satyrs, etc.) but it's set in modern day. This novel really and truly is an amazing work. In fact, I think I need to put it next on my list for a 4th read-through.
Oh, yes, I almost forgot, I have read the Darksword trilogy. I liked it a lot. It was the one about the boy who couldn't use magic in a world where that was a crime punishable by death?
