This underground defensive network dates from the 17th century, when the city was under Spanish rule. They extend to many kilometres and house many gun emplacements. The size of the defnces led Luxembourg to be nick-named the Gibraltar of the North.
Today the site is recognised for its importance by being listed as a Unesco World Heritage site. The complex can be visited and its construction features appreciated first-hand, at the same time as marvelling at the field of fire its guns commanded.