 To ensure Luxembourg's neutrality, the (westward) fortifications of Luxembourg City, known as the "Gibraltar of the North", were to be demolished and never to be rebuilt.To the east, the city was protected by a deep river valley and medieval fortifications that still exist.Dismantling the westward and underground fortifications took sixteen years at a cost of 1.5 million gold francs and required the destruction of over of underground defences and of casemates, batteries, barracks, etc.