Karlsberg Castle (German: Schloss Karlsberg) is a castle ruin on Buchenberg east of Homburg in Saarland, Germany. The castle was constructed from 1778 to 1788 in Baroque and Classical style by Johann Christian von Mannlich, architect and general building director of the dukes of Zweibrücken, by order of Charles II August, Duke of Zweibrücken. The castle was the largest country palace of Europe and served as the residence of the Duke of Zweibrücken.