Schleswig, city (1994 pop. 26,857), Schleswig-Holstein, N Germany, on the Schlei, an inlet of the Baltic Sea. The city's economy is based on the production of food products and leather and on fishing. One of the oldest cities in N Germany, Schleswig was known by c.800. It was a Roman Catholic episcopal see from 947 until the Reformation (16th cent.). The city was the residence of the dukes of Schleswig and (1514–1713) of the dukes of Holstein-Gottorp. It was the capital of Schleswig-Holstein from 1866 to 1917, when it was replaced as capital by Kiel. The fortified Gottorf, or Gottorp, castle (16th–18th cent.) in Schleswig now houses museums of art and early history. The Gothic Cathedral of St. Peter (12th–15th cent.) contains a fine carved reredos by Hans Brüggemann (16th cent.) and the tomb of Frederick I of Denmark.