Fort de Cormeilles - en - Parisis The Fort de Cormeilles - en - Parisis was constructed between 1874 and 1877 as part of a new plan for the defense of Paris developed by General Raymond Adolphe Séré de Rivières , near the town of Cormeilles - en - Parisis . During the Franco - Prussian War , German troops had used the hill upon which the fort sits as a spotting and firing location , ideal for the siting of new rifled guns whose range was increased over old - fashioned smooth - bore guns . These weapons could attack Paris from Cormeilles , so a new outer ring of forts had to be built outside the first ring of forts built in 1840 - 1860 to deny the area to enemy gunners . The Séré de Rivières system was designed as a response to these technological innovations , and to the loss of French territories in Alsace and Lorraine . Description . The trapezoidal fort controls the lower Seine valley and the railway line to Pontoise . The walled fort was surrounded by a ditch defended by two double caponier s and an aileron . Entry was over a laterally - rolling bridge . The Fort de Cormeilles was armed with 64 guns , served by 1132 men , 36 officers and 14 horses , and could operate independently for three months . The new fort cost 3 . 3 million francs d ' or . However , as one of the first of the Séré de Rivières system forts , it was built in brick and stone , with an earthen shield . This construction could not withstand new high - explosive shells that were developed in the 1870s , rendering the fort effectively obsolete without concrete reinforcement . This fort was never upgraded . The fort features a chapel , complete with traceried windows , and a monumental three - story caserne . The façades feature an unusual amount of detail and ornamentation . Six detached batteries and one redoubt supported the fort from the rear : Another battery , the Batterie du Belvedere was located towards the front on the fort ' s glacis , with the Batterie de la Petit Montagne on the opposite side . History . A 1911 project proposed a 75mm gun turret and two machine gun turrets , as well as a protected 155mm gun battery , never realized . In 1914 the fort was equipped with 75mm anti - aircraft guns to counter Zeppelin raids against Paris . The anti - aircraft artillery was upgraded in the early 1930s with four new 75mm guns , replaced by German 20mm guns during German occupation . After 1940 the fort was used by the German occupiers as an ammunition depot for the German navy . Following World War II the fort was used to lodge members of the French Forces of the Interior ( FFI ) , then as a prison until 1956 . From 1956 the fort was used as lodging for prison workers at nearby facilities , such as the Prison de Fresnes . Housing was built on the surface of the fort , and the complex featured a nursery school . The last families , principally North Africans and the family of the fort ' s keeper , left the fort at the beginning of the 1970s . From 1965 to 1997 the fort was used as a commando training center . The Fort de Cormeilles is now the property of the Green Space Agency of the Île - de - France regional government . Preservation work is in the hands of the Association des Amis du Fort de Cormeilles , a volunteer group , which encourages the use of the facility by artists and by television and movie studios . Public visits are available every month and on special occasions . Cinema . The Fort de Cormeilles is regularly used as a backdrop for films and television . Films that used the fort include L ' Autre Dumas , The Blood of Others and The Night of the Generals .