We are at the entrance to the underground passage leading to the northern casemates.
They were built in 1827, under Charles X, to house a "reverse firing battery" intended to protect the rear of the Fort.

In 1994, restoration work began on these casemates, which had largely collapsed. They are now open to the public.

Today, after having been an exhibition and restoration room, it houses the Escape Game "Fort escape Seyne" which can be booked on the website: https://www.fort-escape-seyne.com