Campaign Overview

The tone of the game is historical fantasy with supernatural elements drawn from medieval beliefs rather than modern fantasy - revenants may stalk the night, padfeet may prowl country lanes, cunning-folk make charms to help or hinder, and rare individuals with learning and ambition may call upon occult powers of nature, spirits, stars or demons.
The campaign opens at Christmastide in 1192, as Crusaders return from the Holy Land and it becomes clear that King Richard is missing.
The core campaign area is Nottinghamshire, Derbyshire and the southern part of the West Riding of Yorkshire (see the maps of Northern & Central England and Nottinghamshire), though these are not hard boundaries and characters or campaign focus may move elsewhere temporarily. Characters need not be born in or resident in this area, but should have a reason for being in it and remaining in it for some time. Since Crusaders have started to return to England following the Siege of Acre, a creative player might find a reason for an Arab or African Muslim character to be present.
While I'm clear about the core area of the campaign - it's an area I'm familiar with and get a lot of inspiration from - I'm much more open on the style and focus of the campaign. Some suggestions to discuss in Session Zero:
High Politics: Count John has been spurned as heir in favour of the child Arthur of Brittany and seeks to increase his power base. William Longchamp has been ousted as chancellor and forced out of the country, but seeks a way back to power. Richard the Lionheart is missing, and no one knows where. Queen Eleanor and Richard's regents, including the legendary knight Sir William Marshall seek to preserve the peace and find the missing king. Characters may be nobles, high ranking church officials or secular officials or agents working for one faction or another. Politics, diplomacy and intrigue abound, and open conflict is more likely to be resolved with guile, troops or assassination than individual combat.
Low Politics: As the great men and women of the realm look to preserve or sieze the crown, the barons and leading people of Northern & Central England seek to use the confusion to enhance their power, or at least maintain it. Player characters might be barons or their relatives, advisers and household; county officials or their staff; archdeacons, abbots or abbeses; or leading burghers.
Urban Campaign: Player characters are burghers and residents of a major city such as York or town such as Nottingham seeking to advance their fortunes in the markets and mean streets.
Investigators: The players work for an individual or organisation with sufficient power and resources to employ them (at least part time, if not full time) to investigate wrongdoings and strange events in either an official capacity (such as crown, county officials or baronial officials) or an unofficial capacity.
Adventurers: Similar to investigators, but the player characters between them have sufficient resources to act independently.
Outlaws: Whatever their origins, the players have been convicted of offences serious enough to be branded wolfsheads and banished from law abiding society. Laws no longer protect them, and they may be slain be anyone. They must live off the land and find sufficient allies to shelter and protect them. Perhaps they rob from the rich and give to the poor.

Campaign Notes

Most, but not all, NPCs will be of English, Anglo-Danish, or Norman origin, but as Fitzstephen noted, merchants from "every nation under heaven" are found in London. Player characters who are obviously foreign may arouse curiosity outside ports and major trading towns, but will not be treated with more suspicion than any other stranger. As noted above, characters who do not speak English will be at a disadvantage.
A wave of anti-Semitic riots in early 1190, culminating in deaths of hundreds of Jews in York in March 1190, has increased official and common sympathy for Jews. John Marshal, brother of the famous knight Sir William Marshal, was stripped of the office of Sheriff of Yorkshire for failing to protect York's Jews from the mob. While Crusaders may have left for the Holy Land to fight Muslims, many have realised that the Europeans who have lived alongside Muslims in the Crusader states for nearly 100 years do not share these prejudices, and Muslims, Jews and Christians live together as they do in Andalusia and Sicily. Many Crusaders disapproved of King Richard's execution 2,000 Muslim prisoners of the Acre garrison at the Massacre at Ayyadieh.
King Richard's mother, Eleanor of Aquitaine, Queen-Consort of England, has defined a new paradigm for noble women, and has helped popularise Occitan ideas of chivalry and courtly love. In her younger days she accompanied her first husband on Crusade, where she and her companions dressed in armour in imitation of Amazons. Her daughter Marie, Countess of Champagne (and currently its regent) has created Courts of Love, where women try lovers for supposed offences against love.

Inspirational viewing or reading

TV & Films

Robin of Sherwood - 1980s British TV series featuring Michael Praed (and later Jason Connery) as Robin Hood with pagan inclinations.
Patrick Bergin's Robin Hood - 1990 British film attempting a more historical Robin Hood.
The Lion in Winter - 1968 film featuring Peter O'Toole as Henry II and Katharine Hepburn as Eleanor of Aquitaine and Antony Ho;pkins as Richard the Lionheart, set in 1183 - about 10 years before the start of this campaign.
Kingdom of Heaven - 2005 film featuring Liam Neeson, Eva Green and Orlando Bloom. Despite its odd changes to the characters of Balian d'Ibelin and Guy de Lusignan, it gives a reasonable idea of events leading up to the Siege of Acre, including the Battle of Hattin.

Novels

Ivanhoe - Sir Walter Scott's romance isn't terribly good history, but is set in the time and place of the campaign, and it's a classic.
The Queen's Man - One of Sharon Kay Penman's better books features an agent of Eleanor of Aquitaine investigating the dissappearance of King Richard in January 1193, the very time this campaign starts.
Crowner John series - Bernard Knight's excellent stories about the investigations of one of the first royal coroners are set in Devon from 1194 to 1196.

History

England under the Norman and Angevin Kings - Robert Bartlett's masterly look at the period is chunky, very readable and stuffed with colourful facts about politics, society and the medieval worldview.
England Without Richard - JT Appleby's year-by-year look at events in England in the reign of Richard is a little dated, but so detailed I should probably mark it with spoiler warnings.