The Ruined Castle built between 1779 and 1781 close to the spa baths, although separated on its artificial island behind gnarled oaks, is the earliest pseudo-medieval castle on the European continent to have been purpose-built for a landscaped park in the form of a ruin. In fact, it served Wilhelm, the invested heir to Hesse-Kassel, as a summer residence. Its apparently dilapidated tower has been deliberately designed to bewilder the visitor, for it accommodates an elegant apartment on its ground floor and a magnificent domed hall with portraits of ancestors by Anton Wilhelm Tischbein on its upper floor.