Forts were used in France to protect important towns or cities. Some of the most important forts such as Douaumont at Verdun, were considered to be impregnable. The fort was protected by two layers of concrete 1.2 metres thick with 5.4 metres of earth on top. The fort was also defended by a seven-metre deep moat and 30 metres of barbed-wire.
During the German Verdun Offensive in 1916 over 22 million shells were fired on the 15 forts defending the city. The French were horrified when Douaumont was taken on 25th February.
In July, 1916, the Germans were forced to transfer troops to defend their front-line at the Somme. The French now counter-attacked and General Charles Mangin became a national hero when Douaumont was recaptured on 2nd November, 1916.