L'îlot des Capucins is located at the entrance to the Brest Narrows, on the Quélern peninsula. It's a large schist rock, about 150 m long and 60 m wide, in an admirable geographical position. Its name comes from a nearby rock resembling a monk. Its strategic location had already been noted under Vauban, but it was not until 1848 that this small piece of rock came into its own. The islet was fortified in 1848 and underwent numerous transformations. To set up a battery on this site, part of the rock had to be razed by hand, first the north-western zone to make it suitable for coastal batteries, then the opposite zone to protect it from offshore fire. Only the central ridge will be retained as a protective mask for the barracks. Construction of a longitudinal barracks between 1847 and 1849 to house 62 men, a food store, a cellar, a cistern, a kitchen and a room for the postmaster.
It was not until 1859-1860 that a bridge was built to link the Capucins to the mainland. The fortified system encompassed several so-called "high" batteries built at the top of the cliff, some 500m to the south, which included a battery of 4 mortars of 300mm. On the point itself, a 100mm, then 240mm and 320mm battery, with annexes and terraced powder magazines (1880-1885). 1888 saw the start of the drilling of the rock-breaking battery on the Capucins islet and the construction of a low battery for 4 rapid-fire 47 mm guns. 1890, the powder magazine powder magazine. The plan to install an under-rock breach battery is based on 2 320 mm guns Model 1870-1884, identical to those fitted to the batteries in the Brest Narrows, as at Fort Robert, Cornouaille, Pourjoint (which was modified by the Germans by plugging the firing ports to fix 2 pieces of 50 mm KWK under casemates), pointe des Espagnols to name but a few examples. The original idea was to close the gully with a low, powerful shot, in coordination with the identical work at the pointe du Minou on the other bank. 1890 1892 a searchlight station with a factory for generators was built.