The castle of Ploen is the only well preserved castle in Schleswig-Holstein, which was built in attitude. The first duke of Schleswig-Holstein-Sonderburg-Ploen, Joachim Ernst, built it in between 1633 and 1636. The ducal residence castle is deemed to be the most important renaissance new building during the thirty years’ war. The brick-lined castle is three winged structured and got his white façade not until the 19th century. Even if there is not much left of the original equipment it is still one of the premium architecture artworks of Schleswig-Holstein. During the Weimar Republic from 1920 to 1933 the castle of Ploen was used as a state-run educational establishment with humanistic educational ideals. In 1933 the “Nationalpolitische Erziehungsanstalt” moved into the castle. From 1946 till 2001 the castle was used by a state-run residential school and since 2002 the castle is home of the Fielmann-Academy, a non-profit society with learning opportunities for the whole optically branch of business.