Career
In 1202, he became a Cistercian monk at the abbey of Fontfroide, Narbonne, and he was confirmed as Apostolic legate and first inquisitor, first in Toulouse, and afterwards at Viviers and Montpellier. In 1207 he was in the Rhone valley and in Provence, where he became involved in the strife between the count of Baux and Raymond VI, Count of Toulouse. Castelnau was assassinated on 15 January 1208, quite possibly by an agent of Raymond.
He was beatified in the year of his death by Pope Innocent III, who held Raymond responsible.
The relics of Pierre de Castelnau are interred in the church of the ancient Abbey of Street-Gilles.