Laon is built upon the limestone quarries it mined over the centuries. The underground passageways that served as a prison in the Middle Ages were later part of the town's defence system under Louis-Philippe. The citadel was used as military quarters and a hospital for German troops during the First World War. Beneath the citadel, which is now an administrative building, traces of these miles of passageways remain, spread over three different levels. An immersive audioguided tour takes visitors on a journey through the depths of the city.