The Fort Pierre-lift (known locally as the "Citadel") is on the Ile d'Yeu. It was built in 1858 to 1866 and was designed to house four hundred soldiers and their officers. From 1871 , the fort served in turn as a barracks and prison. Philippe Petain , former head of the Nazi Vichy government , whose death sentence was commuted to life imprisonment, to serve his sentence from 1945 to 1951 and died there.
http://www.marechal-petain.com/versionanglaise/prisonnier.htm