Beneath the Montée de Clausen, the clifftop site of Count Sigefroi’s once-mighty fort, the Bock Casemates are an atmospheric honeycomb of rock galleries and passages initially carved by the Spaniards from 1644 onwards. They were extended by French engineer Vauban in the 1680s, and again by the Austrians in the mid-18th century. Over the years the casemates have housed everything from garrisons to bakeries to slaughterhouses; during WWI and WWII they sheltered 35,000 locals. Kids will adore exploring the passageways.