Fort Douaumont was the largest and highest fort on the ring of 19 large defensive forts protecting the city of Verdun, France since the 1890s. It has a total surface area of 30,000 square metres and is approximately 400 metres long, with two subterranean levels protected by a steel reinforced concrete roof 12 metres thick resting on a sand cushion. These improvements had been completed by 1903. The entrance to the fort was at the rear. Two main tunnels ran east-west, one above the other, with barracks rooms and corridors to outlying parts of the fort branched off of the main tunnels. The fort was equipped with numerous armed posts, a 155 mm rotating/retractable gun turret, a 75 mm gun rotating/retractable gun turret, four other 75 mm guns in flanking 'Bourges Casemates' that swept the intervals and several machine-gun turrets.
By 1915 the French General Staff had concluded that even the best-protected forts of Verdun could not resist bombardments from the German 420 mm Gamma guns. These newly deployed giant howitzers had easily taken several large Belgian forts out of action in August 1914. As a result, Fort Douaumont and other Verdun forts were judged ineffective and had been partly disarmed and left virtually undefended since 1915.
On 25 February 1916, Fort Douaumont was entered and occupied without a fight, by a small German raiding party comprising only 19 officers and 79 men. The easy fall of Fort Douaumont, only three days after the beginning of the Battle of Verdun, shocked the French Army. It set the stage for the rest of a battle which lasted nine months, at enormous human costs. Douaumont was finally recaptured by three infantry divisions of the French Second Army, during the First Offensive Battle of Verdun on 24 October 1916. This event brought closure to the Battle of Verdun in 1916.
Today Fort Douaumont is open to the public.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.