Built on a narrow rocky spine, the upper rampart of the citadel of Sisteron forms a walkway dominated by a powerful 13th-century keep. This massive belfry tower housed the municipal bell and clock in the 15th century. The 1402 clock divided into twenty-four hours was replaced in the 16th century by a twelve-hour dial. At the beginning of the 17th century, the clock tower was converted into a military building and intended for prison use. It is possible to visit the reconstitution of the dungeon of Prince John Casimir Vasa, the future King of Poland, imprisoned there by order of Richelieu from February to August 1639.