Luxembourg City is built on and around a sandstone promontory rising above two river gorges, and that geography made it one of the most heavily fortified places in Europe — the 'Gibraltar of the North'. The old quarters and fortifications have been on the UNESCO list since 1994.
The casemates
Beneath the city runs a network of casemates: tunnels, galleries and gun emplacements carved into the rock from the seventeenth century by Spanish, French and Austrian engineers. At their greatest extent they ran twenty-three kilometres; about seventeen survive.
Two sections are open. The Bock casemates, at the site of the original tenth-century castle, are the more dramatic, with openings looking straight down into the Alzette valley. The Pétrusse casemates are larger and less visited. During the Second World War they sheltered tens of thousands of people during air raids.
When Luxembourg's neutrality was guaranteed by the 1867 Treaty of London, the fortress had to be dismantled — a process that took sixteen years. What remains is what could not practically be removed.
The Corniche
The Chemin de la Corniche runs along the old ramparts above the Grund, and the description as 'the most beautiful balcony in Europe' — coined by the Luxembourgish writer Batty Weber — has stuck because it is roughly accurate. It takes fifteen minutes to walk and gives the defining view of the city: the valley below, the Neumünster abbey, the bridges and the rock.
The upper town
The Grand Ducal Palace, in Flemish Renaissance style, is the town residence of the reigning Grand Duke; the changing of the guard happens outside, and in summer the interior opens for guided visits.
Place d'Armes and Place Guillaume II are the main squares, with markets and cafés. The Notre-Dame Cathedral holds the ducal crypt, and the Adolphe Bridge — a stone arch of 1903 spanning the Pétrusse valley — is the city's engineering landmark, now with a pedestrian deck slung beneath the roadway.
Practical
The city is small and the historic core is walkable, though it involves considerable up and down between the plateau and the valleys. Lifts and a funicular connect the levels, and — like all public transport in the country — they are free.
Note that Luxembourg City is expensive: it has one of the highest costs of living in the EU, and hotel prices reflect the business and institutional traffic rather than tourism. Weekends are noticeably cheaper than weekdays, which is the reverse of most destinations.
How long to stay
Two nights covers the city comfortably: one day for the casemates, the Corniche and the upper town, another for the Grund, Kirchberg and a museum. The country is small enough that the capital also works as a base for the whole of it — Vianden, Echternach and the Moselle are all within an hour.