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. https://www.visitluxembourg.com/en/place/castle/bock-casemates-luxembourg