I've been to Luxembourg city a few years ago. It was a day trip from Bruxelles but it was enough to visit the city and explore the Bock casemates, one of the coolest places to visit there.
Luís was there a few weeks ago. He sent me the 1st of these cards and the other was bought by me.
In 963, Count Siegfried built a fortified castle on the Bock promontory, which was soon to become the cradle of the city. In the course of the centuries, on the western side, mighty ring walls were added, which, however, did not foil the Burgundians in their attempt to conquer the city in 1443. The best builder-engineers of the new masters (the Burgundians, the Spaniards, the French, the Austrians and the German Confederation) eventually turned the city into one of the most powerful emplacements in the world, the "Gibraltar of the North".
Its defences were bolstered by three fortified rings with 24 forts, 16 other strong defensive works and a unique 23 km long network of casemates: these could not only shelter thousands of soldiers and their horses, but also housed workshops, kitchens, bakeries, slaughter-houses etc. In 1867, after the declaration of neutrality,the military withdrew from the fortress and during the following 16 years 90% of the defences were demolished. In 1875, the superstructure of the Bock, a tremendous construction, was razed. However, it proved to be impossible to blow up the casemates, without also demolishing part of the city, so the entrances and the key connecting galleries were sealed. In spite of this, 17 kilometres of tunnels remain, often on different levels and tremendous stairways penetrate up to 40 metres inside the rock face. - in: http://www.visitluxembourg.com/en/place/castle/bock-casemates-luxembourg