Begun in 1843, the Fort Moyenne and Fort Supérieur were constructed over a 20-year period. The construction of Fort Moyenne spanned 1846–1862, and Fort Supérieure 1852–1860. The B12 and the underground stairway were also constructed at this time. The purpose of the fort was to lock the access to the Ubaye valley from the Cols, and the fort was so positioned to allow covering fire from the bronze cannons firing round shot that were installed there. With a range of 3-400m, these cannons were able to cover the roads (well, tracks) coming down from the Col de Vars and the Col de Larche, as well as the upper Ubaye valley. Looking from the B12, one can see what an excellent prospect of the valleys this afforded. Cannons were also installed at batteries on the eastern side of the Fort Moyenne and the Fort Supérieur.