The lesser-known partner of the Bock Casemates, carved into the cliffs above the Pétrusse valley. The tunnels were originally Spanish work from 1644, then extended by the French under Vauban after 1684. They are smaller in scale than the Bock but more atmospheric, with the descent passing the old Beck Bastion and the powder magazines.
Reopened in 2021 after a major restoration. Access is by guided tour only (around an hour), starting from Place de la Constitution. Cool inside year-round; the steep narrow stairs are not suitable for very small children or mobility-impaired visitors.