The origins of Tallinn date back to the 13th century, when a castle was built there by the crusading knights of the Teutonic Order. It developed as a major centre of the Hanseatic League. The Historic Centre (Old Town) of Tallinn is an exceptionally complete and well-preserved medieval northern European trading city on the coast of the Baltic Sea. The city developed as a significant centre of the Hanseatic League during the major period of activity of this great trading organization in the 13th-16th centuries. The combination of the upper town on the high limestone hill and the lower town at its foot with many church spires forms an expressive skyline that is visible from a great distance both from land and sea.