Named after General Senarmont, Fort Bessoncourt was erected between 1883 and 1886 to block the roads to Colmar and Basel and the Mulhouse railway line, and to cover the forts of Roppe and Vézelois. It is a large limestone masonry fort, pentagonal in plan, designed for about 650 men and about 30 artillery pieces. It is representative of the forts of the 1880s, in which the artillery for distant action and that for close defense were assembled on the rampart bordering the ditch, while the infantry took up position above the caponiers and on the roof of the central barracks. But shortly after its construction, it is already outdated and needs modernization. It is one of the few forts in the square to have constantly kept pace with technical progress: in 1888, a concrete shell was poured over a third of its central barracks: in 1894-1895, most of its artillery being dispersed in surrounding batteries, it became an infantry support point and underwent a complete overhaul. In 1908-1909, two turrets for 75 guns and two machine gun turrets were installed. During the First World War, the fort served as a hospital, but from 1917, fortification work resumed with the digging of deeply buried rooms and galleries connecting the fort with the outside.