Pétrusse Casemates

The square you're standing on is not really a square. Place de la Constitution is the courtyard of a gun emplacement — the Beck Bastion, thrown up in 1644 when this city belonged to the Spanish crown. It was drawn on the plans as Saint Jean, but everyone named it after Jean Beck, a Luxembourger who rose to govern the province. The Gëlle Fra now stands roughly where its cannon once pointed out over the valley.

What you can't see from up here is the inside of it. The rock beneath your feet is hollow. When the Spanish raised the bastion they began cutting galleries down into the cliff, and after Vauban took the city for Louis XIV in 1684 French engineers went further, carving gun chambers and stairways into the stone above the Pétrusse. Soldiers could move, sleep and fire without ever showing themselves on the surface.

These are the Pétrusse Casemates, and they are the other half of the system you may have already met at the Bock. Where the Bock tunnels defend the Alzette side, these hold the southern approach, the softer flank where an attacker would most likely have come.

Most of the fortress above ground was pulled down after 1867, when the powers of Europe declared Luxembourg neutral and ordered its walls demolished. The tunnels survived because blowing them up would have brought the streets and houses above down with them. In 1994 UNESCO listed them along with the rest of the old quarters — a fortress remembered mostly by its underside.