A most intriguing attraction of Luxembourg is its Bock Casemates. The Casemates entail a 21 kilometer network of underground passages. Entrance to the Casemates is granted at the Bock Cliff. The Casemates tourist attraction is part of the UNESCO World Heritage site. The original casemates spanned 14 miles of defensive passageways. The fortifications date back to 1644, earning Luxembourg the title of "Gibralter of the North". Some of the passages run an impressive 131 feet underground. The original purpose of the Casemates was to serve as a defensive passageway. The first tunnels were built during the Spanish domination. Forty years later, the tunnels were expanded by the French military engineer and fortress builder, Vauban. At its peak, the network of tunnels accommodated thousands of defenders by housing its occupants along with numerous horses, artillery workshops, slaughterhouses, kitchens, bakeries, and equipment. Subterranean defensive passages were dug out one several different levels of the casemates to better facilitate the army. Overall, the fortifications were built over a time span of nine centuries. The Casemates were at their prime during the 17th and 18th Centuries. After the French Revolutionary Wars, when Luxembourg managed to hold off French sieges for seven long months, the Bock Casemates earned its reputation as "Gibraltar of the North". This title was given after French politician and engineer Lazare Carnot called the city "the best fortress in the world except Gibraltar". In 1867, the fortress was dismantled and a mere 17 kilometers of the casemates were reserved. Several years later, in 1933, the Bock Casemates were opened to the public and today serves as a historical site.