Glücksburg Castle (German: Schloss Glücksburg, Danish: Lyksborg Slot) is a water castle (German: Wasserschloss) in the town of Glücksburg, Germany. It is one of the most important Renaissance castles in Northern Europe. It was the seat of the House of Schleswig-Holstein-Sonderburg-Glücksburg and was also used by the Danish kings. Located on the Flensburg Firth, the castle is now a publicly owned museum, no longer a residence of the ducal family. The castle was built between 1582 and 1587 by Nikolaus Karies for John II, Duke of Schleswig-Holstein-Sonderburg (1545–1622) on the site of the former Cistercian monastery of Ryd Abbey, the building materials of which were partly reused in the castle. The grounds of Ryd Abbey were later flooded to create a large pond that almost completely surrounded the castle.