The specific topography of Besancon, situated in the meander of the River Doubs, represented an important challenge for Vauban.The result formed, with the 19th century fortifications, a grouping of the first order. The citadel, planted on the rocky spur that closes the meander of the Doubs, was built between 1668 and 1683. Acquired by the town in 1959, it is the only Vauban citadel open to the public, housing museums, exhibition space and a zoo. Fort Griffon was built by Vauban between 1680 and 1684 on Battant enceinte. The Universitaire de Formation des Maitres institution is installed there. The enceinte of the loop of the Doubs and its bastioned towers were built in 1687. The towers received their current roofs in the 19th century. The group of the urban enceinte was built between 1677 and 1695. The barracks in the town were built between 1680 and 1848. Five advanced lunettes "a la d'Arcon" were built after 1791, although only three remain. In the 19th century, detached forts encircled the fortified town.