Intermediate infantry fortification dug out of the rock towards 1890, it marks the change with forts built between 1874 and 1885 (smaller and more compact) and introduces the type of layout to be used for the Méroux construction: a trapezoid where the ditches are protected by means of counterscarp covert ways.
Although this fort was principally built to fight against infantry infiltrations, it was equipped, in addition to the 4 long-range artillery pieces (to cover the Route de Lure), with four small-calibre cannons. The access was via the ditch; the rooms were all underground and designed for more than 250 men. Concrete was not used a lot as there is rock on site which replaces concrete advantageously. The only parts to be in concrete are the counterscarp covert ways, the casemates defending the entrance and the shelters on the upper part.
- Secondary type: castles and defense sites