Re: Deep Medieval RPGing
Well, the Dark Ages were largely a myth, FWIW. I put Harn largely in the early medieval period, with a brief digression into late Republican Rome, and with some early high medieval, particularly in terms of the power of the guilded classes. It's a hodge-podge to be certain, but for my money (and I spent far too many years studying medieval history), it captures the feel of the age better than most other games.
It was quite possible to play a straight historical version of Chivalry & Sorcery, though the game is so crunchy as to make me use the Hero System for a pillow.
While Pendragon focuses on the medieval romance as a default, it's quite easy to play down those aspects of a campaign.
I've known people who've used Ars Magica without the magic. Because of the way the game is designed, with three tiers of player characters (Mages, Companions, and the communally-played Grogs) it's not that hard to strip out the mages and have a solid medieval experience.
The Riddle of Steel has a default setting that's a bit more "medieval fantasy," but to be honest, the magic system is very problematic and quite easily omitted, leaving behind a solid system for exploring any medieval or renaissance setting you'd like, with an emphasis on realistic swordplay.
Others mentioned A Game of Thrones, which is either a d20 or Tri-Stat adaptation of George R. R. Martin's series, "A Song of Ice and Fire." The d20 rules are the best gritty d20 adaptation I've ever played and the setting (especially in the early books) is very much a low or no magic high medieval setting.