From 1994 to 1997 and three encore games in 1998 to 2001 I ran a 2nd Edition game based in a world similar to 14th Century Europe. I called it Grymwurld: A Dark Mirror™. I used a lot of material from the Historical Campaign Sourcebooks and borrowed liberally from Chivalry & Sorcery, Pendragon, Ars Magica, and Vampire: The Dark ages.
It was the most enjoyable campaign I've ever ran in my 30 years of D&D. A lot of it had to do with the players I was blessed to have in the game but part of it was sucessfully interjecting a lot of Late Mediæval "look & feel" or "fluff" into the campaign. Off the top of my head are some things to consider:
The Church. An article in Dragon magazine in the late '80s presented the Roman pantheon with an intriguing idea — that a lot of the Greek deities who were rather Chaotic were recast as Lawful in accordance with Imperial Roman society. So I ran with it. The Celestial Imperium has as it's emperor Jupiter who shares power to some extent with his brothers Neptunus and Pluto (a.k.a. Dispater, a.k.a. Orcus, a.k.a. Hades). Mars is the celestial general, and so on. In response to the barbarians at the gate and the collapse of Namora (Rome), the Celestial Imperium further organized itself into a form comparable to the historical church.
Clerics are called Chaplains in Grymwurld: A Dark Mirror™ and are dedicated to Mars. Most of them are Lawful Evil and engage in Crusades against the forces of Chaos on a regular basis. They cannot turn undead, are not granted curative magic, but do gain martial weapon proficiencies. Miracles (spells) cannot be invoked (cast) unless the chaplain removes his gauntlets, helm, and sets aside his shield. Most chaplains fight with the greatsword.
Black Friars are in 3e terms cloistered clerics dedicated to Pluto. They are called "black" because they wear black robes. In 2nd Ed. I made them human multiclassed priest/wizards who specialized in necromancy and forgo evocation. The cult of Pluto is organized much like a Benedictine order who pray for the souls of the departed and perform funeral rites. The Black Friars primary role is to seek out undead and destroy them. The undead are an abomination, a theft from the Lord of the Dead. However, Black Friars cannot turn or rebuke undead.
Navigators are "priests" of Neptunus but in actual practice monopolize the sea trade and are hardly pious at all. In game terms they were wizard/priests who specialized in sea magic.
Knights of Mars were LE or LN "paladins." Mostly paladins by the book but with a focus against Chaos rather than Evil, i.e. Detect Chaos, protection from chaos, etc. In 3e they would Smite chaos. Perhaps the Paladin of Slaughter might be appropriate. They served the Chaplains of Mars in their Crusades against Chaos.
Death Knights were defenders of the Black Monks and in game terms effectively Crypt Rangers.
Astrologers were "priests" of Apollo but had become corrupted to the point of serving merely as advisors to lords. In game terms they were cleric/wizards who specialized in divination.
I should also note at this point that in addition to banning turn/rebuke undead, all cure spells were empathic. That is, the caster had to absorb the damage being healed. Whenever Sir Stephen would ask Br. Casticus for a cure spell, the response was usually 'Do you have positive hit points? In that case, you'll live.'
The Culdeans a.k.a. Green Friars or Green Monks were a cult of pacifists. Inspired by the historical Celtic Church and the Grail Christians of Pendragon. They were never proficient in any weapons and could never invoke a miracle that would harm another creature. However, they could do something nobody else could . . . non-empathic curing!
Benefit of Clergy. A person who could read and write in Latin could claim the benefit of clergy. This benefit exempted a person from secular authorities (like diplomatic immunity) and were only subject to the Ecclestical court. It is also a crime punnishable by death to harm a cleric. However, the "cleric" could not make this claim while armed and/or armoured. Doing so forfeited their benefit. Consequently, Chaplains almost never claimed the benefit but all other "clerics" did. Sometimes students would abuse this benefit by getting into tavern brawls with other students and not make reparations to the innkeep.
Nobles are ransomed and treated like guests while commoners are tortured. This lead to a humourous situation where the party had been captured. The noble PCs were treated hospitably while the commoner PCs were tortured. The players of the noble PCs mercilessly teased the players of the commoner PCs. Well.... you had to be there. It really was pretty funny.
Spellcasting was either scholarly (read from a grimoire/prayer book with 1 step longer casting time) or witchcraft (chaotic wild-magic wizards). There was a half-elf in the party named Dyvynarth who practiced witchcraft but pretened to read from a grimoire each time. Sometimes he read from the book upside down because he was illiterate but then again so were the knights in the party. This lead to the funny situation: Sir Stephen "Hey! You better be reading that spell from a book or I'll have to burn you at the stake!" Dyvynarth "Oh, of course I am (turning the book rightside up)!"
With the lack of evocation spells and spellcasting taking 1 step longer (segments became rounds, rounds become turns), most spellcasting focused on prep spells and not combat spells. Unless of course the PC was a witch but then had a chance of getting a wild surge.
Culturally, all spellcasters got their university degrees in divinity. Those who decided to explore profane magic would get their doctorate in magica and swear a Hermtic Oath. The Hermetic Oath was modeled on the Hippocratic oath basically promising to use magic to help and not harm people.
Jealous protection of secrets. An important piece of fluff. Knowledge was hoarded and guild scecrets were protected with death threats. Getting training from NPCs without joining a guild was very very difficult. Most spellcasters had to rely on research or treasure to learn spells.
Heresy was not tolerated. Anyone who detected as having a Chaotic alignment would be executed on the spot. This put the PCs in an awkward postion sometimes of having to decide between Chaotic good outlaws and Lawful evil church and state.
The Black Death. As a parallel to the bubonic plague, a Romero-esque zombie plague sprouted up. With cure disease being empathic and turn undead unavailable, this zombie plague succeed in destroying 1/3 of the population of the known world. The way it was destroyed was not through attritition — the PCs and the govt. tried this and failed — but by locating and destroying the source of the plague — a lich. If the PCs had not succeeded in destroying the lich more would've died. Note that the PCs were not the only ones searching for the lich nor were they the only ones who found it. Just the ones who lived to tell the tale.
Dwarves were treated much as the Jews in the Middle Ages. Dwarves possesed a near-monopoly on mines, metallurgy, and eventually banking. That commercial success plus their obviously physical differences and clannish nature inspired frequent jealousies in the Humans. Sometimes a monarch would drive the dwarves out of his kingdom instead of paying his debts. Dwarves would get accused of poisoning wells, kidnapping babies, and all the other injustices that the Mediæval Jew faced.
An idea I have but never tried to is represent the Mongols as centaurs. Imagine a brilliant and charismatic Half-Fiend/Centaur uniting all the centaur tribes of the steppes.
Or a brilliant and charismatic half-ogre or Half-Fiend/Ogre uniting all the ogre tribes much as Atilla did?
I also forgot to mention that most nobles are Lawful (evil tendencies) and most commoners are Neutral (good tendencies). Outlaws are Chaotic (or are Chaotics are outlawed?). Although Raven Crowking made some eloquent statments reminding me of the collective nature of Mediæval rural life. So perhaps the commoners should be more lawful. By the way, if you look at heraldry one of the lessons to learn is that the nobility saw themselves are being eternal and unchanging. That there was always and will always be a ruling class. That the noble houses have long lineages. That the status quo must be upheld. Nevermind that this was not reality, it was the ideal. Also that commoners don't long for an anarcho-syndicalist-collective but rather for just and merciful lords.
PCs should come from the ranks of the lower nobility and be the youngers sons and daughters. Rather than giving into pressure to marry well they instead seek the life of a knight-errant and go forth to sow their wild oats. Althogh the rogue may be a low born serf who ran away from home.
I adapted the troupe concept from Ars Magica and WoD in that each player had two characters, one noble (knight or "cleric") and one commoner (rogue, sergeant-at-arms, dwarf, half-elf, or half-ogre). In 3e this could be simulated by starting the PCs at 2nd level and giving them the leadership feat for free. By doing so, the players could engage in noble activities with their noble characters and base activities with their common characters. The spellcasters ("clerics") would cast their prep spells and the knights would wade into combat.
There was much, much more of course but this should suffice for now.
As for d20 resources I recommend taking a look at:
- Alea Publishing: A Question of Loyalty: A Guidebook to Military Orders, A Question of Honor: A Guidebook to Knights, and Honor & Corruption.
- RPG Objects: Legends of the Dark Ages and Legends of Excalibur
- Atlas Games: The Black Monks of Glastonbury (adventure)
- Green Ronin: The Medieval Player's Handbook
- Expeditious Retreat Press: A Magical Medieval Society
- Avalanche Press: Noble Knights
- Mongoose Games: CONAN RPG
- Malhavoc Press: Iron Heroes