High on a peak in southern France is Montségur, the last outpost of a medieval sect, the Cathars. Many Cathar settlements were in towns and villages. The Cathar's name means "puritans" sometimes called Albigensians after the town of Albi where they were particularly strong. Their religion originated in eastern Europe but flourished in France and Italy in the twelfth and thirteenth centuries until they were destroyed in a crusade against them in 1208. The ruins of their religious centers are found in several places in southern France.

The Cathars taught that there were two gods and denied the death of Christ, the sacraments, and the Resurrection, which made the medieval Church their bitter enemy. The Cathars were led by a sect called the Perfect, and lived a common life. They ate no meat because it was the product of sexual generation. Their only prayer was the Lord's Prayer, recited up to 40 times a day. The Cathar faith was passed on within families under the influence of grandmothers, who were the guides of people's consciences.

When the crusade armies marched against the Cathars, in one town alone, 20,000 Cathars were slaughtered after they had surrendered. The Albigensian Crusade or Cathar Crusade (1209 - 1229) was a 20-year military campaign initiated by the Roman Catholic Church to eliminate the religion practiced by the Cathars of Languedoc, which the Roman Catholic hierarchy considered apostasy. The Albigensian Crusade had a role in the creation and institutionalization of both the Dominican Order and the Medieval Inquisition. In 1232 the surviving Cathars moved their headquarters to Montségur, a castle built by Esclarmonde, one of the holiest of the women Perfects set on a limestone outcrop in the Languedoc-Midi-Pyrénées Mountains. The castle was able to survive until forced surrender in 1244. Montségur has been located as one of the Holy Grail, the cup that Jesus jused at the Last Supper. Despite the fact that sun worship would have deeply offended Cathar beliefs, Montségur has become the focus of New Age cults, vegetarians, and feminists. Members of The Church of Jesus Christ of Latter-day Saints (Mormons) believe that this foretold apostasy, "The Great Apostasy," began with the death of the early apostles and continued into the early nineteenth century. Flavius Claudius Iulianus, Roman Emperor (361-363), was raised as Christian, but rejected this faith upon gaining the purple. His Christian enemies called him apostate, and therefore he is still widely known in English as Julian the Apostate.

After the failure of the uprising against the French invaders, the defeat of Henry III of England by Louis IX of France, the events at Avignonet, and the capitulation of Ramon VII, all in 1243, the Council of Beziers decided to destroy the last vestiges of Catharism. The Cathar sympathisers responsible for killing theInquisitors at Avignonet were known to have come from Montségur. The present fortress ruin at Montségur is not from the Cathar era.

The original Cathar fortress of Montségur was entirely pulled down by the victorious French Royal forces after its capture in 1244. The castle was besieged later in 1443 by Hughes des Arcis, Seneschal of Carcassonne for the King of France. It was gradually rebuilt and upgraded over the next three centuries by Royal forces.