Ruins of the old castle and the vast underground system of passages and galleries known as the casemates continue to be a major tourist attraction.
It was in 963 that Count Siegfried, in search of a site from which he could defend his properties, obtained the Bock and its surroundings from St Maximin's Abbey in Trier in exchange for the land he owned at Feulen in the Ardennes to the north.
After the consent of Emperor Otto I had been obtained, the deed was signed by Viker, Abbot of St Maximin's, on 7 April 963.
[4] Over the centuries, Siegfried's fortified castle on the Bock was considerably enlarged and protected with additional walls and defences.
[3] As time passed, the fortifications needed to be adapted to new methods of war based on increasingly strong firepower.
[6] A little later in 1684, on behalf of Louis XIV, Vauban succeeded in capturing the city of Luxembourg during a month-long siege under which the Bock fortifications were completely flattened.
[6] Thereafter Vauban, perhaps the most competent fortification engineer of his day, undertook major additions to the defences, realizing that underground passages and chambers were just as important as the surface installations.
Branches leading off on either side were equipped with no less than 25 cannon slots, 12 to the north and 13 to the south, offering considerable firepower.
This led the French politician and engineer Lazare Carnot to call the Luxembourg fortress "the best in the world, apart from Gibraltar".