Hmmmm, yes. It is an interesting but challenging sort of game to run. We've been experimenting here. I note a few things that have been challenging, and a few tentative conclusions:
1) Disparate character types in a medieval setting. How do they relate to each other? Do you make all the PCs knights, or are some of them other types of character? Medieval society is quite stratified and classist. How does a merchant or some other non-knightly character fit in with the concept. Clearly the easiest solution is to make all PCs knights. The other solution is to play a game with more political intrigue or other elements and downplay the 'knightly adventure' part.
2) Arthurian type tales are very abstract. I read a bunch of medieval literature to get an idea of the sort of elements that these romantic tales contain. What you immediately find when you start building a setting and playing is that they are very abstract 'high fantasy' kind of stories. Rarely does practical reality intervene into the stories. You really need to avoid 'grittiness' if you're going to be true to the stories. While now and then practical considerations pop up in some of the literature they are at best plot devices.
3) The action is highly individualistic. While there are times when several characters act together in these stories most of the time a single main character is acting alone. The scope for a party of PCs acting together is limited, and when it does exist most of the characters are likely to have bit parts. This kind of thing could be handled by making the campaign rather abstract, so you let the players describe what they do for long periods of time in general terms and develop out a plot where each PC does 'knightly stuff' that isn't handled in detail while the plot develops to a point where it is logical to have them come together (the DM will of course help engineer this) and then they have an adventure. Afterwards the PCs will once again go their separate ways until the next time.
Alternately you can simply create a 'party', but it is a rather artificial construct within this genre.
It turns out to be quite a challenging type of game to run. Not much like your more typical D&D-esque FRPG setup with parties of adventurers.
I think mostly I'd highly structure the campaign around some sort of story arc that demands a lot of derring-do and concerted action by the PCs and gives them a good reason to form a logical faction. Center it on knights and keep it fairly abstract. Railroading in this type of game I'd think could often be fluffed as being the machinations of fate/gods/whatever. Yes, it is odd how the PCs constantly find themselves together at the right time and place for interesting things to happen, but that's the way the fates work! hehe.
Anyway, we're still experimenting heavily. I'd love to hear more ideas too.
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