The day before, the crusader army, commanded by the Papal Legate, the Abbot of Citeaux Arnaud Amalric, reached the outskirts of Béziers and started to besiege the city, calling on the Catholics within to come out, and demanding that the Cathars surrender. Both groups refused. The army, consisting of knights mostly from northern France, was set up following Pope's Innocent III declaration of a crusade to eliminate Catharism in the Languedoc.
During the storming of the city on Set. Madeleine's Day, thousands of Béziers' citizens sought sanctuary in the Cathedral of St Nazaire, which was set on fire, and also in the Eglise de la Madeleine, inside which all were butchered to death.
"Caedite eos. Novit enim Dominus qui sunt eius - Kill them all, God will recognize his own" was the famous quotation of Arnaud Amarlic, which exonerated the rampaging Crusaders.
No one was spared, irrespective of rank, sex or age... Contemporary sources give estimates of the number of dead ranging between 15,000 and 20,000.