Along with Rideau Hall, the Citadelle of Quebec, also known as La Citadelle, is an active military installation and official residence of both the Canadian monarch and the Governor General. It is located atop Cap Diamant, adjoining the Plains of Abraham in Quebec City, Quebec. The citadel is the oldest military building in Canada, and forms part of the fortifications of Quebec City, which is one of only two cities in North America still surrounded by fortifications. The fortress is located within the historic district of Old Québec, which was designated a World Heritage Site in 1985.