During the 1930s, the Séré de Rivières fortifications of the nineteenth century dominating the Roya river became ineffective against aerial fire. They were replaced in the context of implementation of the Maginot Line.
On the Maginot line, surface constructions are limited to concrete blocks. Their number depends on the size of the structure. Located at the back of the fire zone, an entrance block gives access to a gallery leading to the underground barracks. If a road serves it, this block can have a large door for vehicles loaded with ammunition and equipment. In this case the gate is protected from a drawbridge ditch.
One or several blocks used as shooting rooms insure the defense. Located on the slab of a dominant block, a steel bell allows observation in addition to the close defense. These battle and observation blocks are scattered on the groundside fire zone. Wells and galleries connect all the blocks, sometimes more than 30 meters deep under the rock creating a shelter from bombing.