Notre-Dame du Bourguet Cathedral is an essential part of the heritage of Forcalquier, of which it is the most visited site. Built between the 12th and 17th centuries, it was erected as the co-cathedral of the diocese of Sisteron in 1408. Its central nave, choir, transept, and the base of its bell tower date from the beginning of the 13th century. The steeple is from the 16th century, and the side naves and the second floor of its bell tower were built during the 17th century. The cathedral's unprecedented mixture of Romanesque and Gothic constitutes the first attempt at adapting Gothic art in the Pays d'Oc.