This
staircase leads to the casemates which are added to the fortress
under Louis XVIII in order to strengthen its defense system.
They consist of a subterranean passage that runs through the
entire width of the fortress and opens at both ends on two small
vaulted rooms.
The south wall of the underground passage is pierced, every
two meters approximately, of loopholes surmounted by chimneys
which served to evacuate the abundant smoke that emitted the
powder during exchanges of shots.
The walls of each of the two vaulted rooms are pierced with
three loopholes facing each other.
The southern casemates thus allow to put into practice the system
of "cross shot" dear to Vauban.
The southern casemates are only accessible for guided tours.