The citadel of Sisteron crowns the city on almost 25 acres (10 ha) of fortifications. Today's fortress bears witness to several centuries of architecture and history through its works from various periods. The upper rampart (walkway) built on the narrow rocky spine and punctuated by a massive keep dates from the 12th or 13th centuries. It forms a coronation line to which was adapted in the 16th century a layer of bastioned structures attached to the rampart that had surrounded the city since the 14th century. The north face of the citadel was subsequently extensively redesigned in the middle of the 19th century.