Nicknamed al-Bahjah or Alger la Blanche ("Algiers the White") for the glistening white of its buildings as seen sloping up from the sea, it is situated on the west side of a bay of the Mediterranean Sea. The city name is derived from its location on the slopes of the "Sahel", a chain of hills parallel to the coast. The modern part of the city is built on the level ground by the seashore and the old part, the ancient city of the deys, climbs the steep hill behind the modern town and is crowned by the casbah or citadel, 400 feet above the sea. The casbah and the two quays form a triangle. There are many public buildings of interest, including the whole casbah quarter, Martyrs Square (Sahat ech-Chouhada, the government offices (formerly the British consulate), the "Grand", "New", and Ketchaoua Mosques, the Roman Catholic cathedral of Notre Dame d'Afrique, the Bardo Museum (a former Turkish mansion), the old Bibliotheque Nationale d'Alger - a Turkish palace built in 1799-1800 - and the new National Library. The main building in the casbah was build in 1516. A road has been cut through the centre of the building, the mosque turned into barracks, and the hall of audience allowed to fall into ruin. The Grand Mosque  is traditionally said to be the oldest mosque in Algiers. The pulpit (minbar) bears an inscription showing that the building existed in 1018. The New Mosque is in the form of a Greek cross, surmounted by a large white cupola, with four small cupolas at the corners. The minaret is 90 ft. high. The interior resembles that of the Grand Mosque.