Plön Castle in Plön is one of the largest castles in the north German state of Schleswig-Holstein and the only one located on a hill. The former Residenz of the Dukes of Schleswig-Holstein-Plön was built in the 17th century during the Thirty Years War and has had a colourful history in which it has, for example, been a school for military cadets and also a boarding school.
The castle ended the twentieth century as a possession of the state of Schleswig-Holstein, but they could not fund its renovation and so it had to be sold again. Since January 2002 it has belonged to the Fielmann Akademie Schloss Plön and, after extensive conversion, serves as a training and qualification centre for a branch of optometry. After being inaccessible to the general public for many years when it was a boarding school, its new owners have opened it to a limited extent to visitors.