Passerelle

Locals just call this one "Al Bréck" — the Old Bridge — and it earns the name: this viaduct across the Pétrusse valley opened in 1861, more than forty years before its more famous neighbour, the Adolphe Bridge, was even built. Before it, the only way across at this height was a wooden footbridge — a passerelle — and the stone bridge simply inherited the name.

It exists because of the railway. The station opened in 1853, and it was put deliberately far from the city, out on the plateau across the valley, so that it could never be used against the fortress. That left the problem of reaching it. Two engineers, Grenier and Letellier, drew this answer, and a British firm, Waring Brothers, built it between 1859 and 1861: twenty-four arches, 290 metres long, 45 metres above the stream. Look along it from either end and you'll see it curves. That was on purpose — the fortress was still armed, and a straight bridge would have blocked the line of fire from the Holy Ghost plateau. A curved one let the cannon shoot straight along it.

It's still doing its job today, widened twice — from seven and a half metres to fifteen in 1960, and again in 2018 — and carrying road traffic and pedestrians on the same route it has for over 160 years. Stand underneath it, down in the Pétrusse park, and the scale becomes obvious in a way it isn't from the road above: a long procession of arches marching across the whole valley, with the fortress city rising on the ridge beyond. It's a good spot to appreciate how much of Luxembourg's old town sits on a cliff — such a naturally defensible position, and such an engineering headache whenever the city needed to grow beyond its walls.