Glücksburg Castle (German: Schloss Glücksburg, Danish: Lyksborg Slot) is a water castle (Wasserschloss) in the town of Glücksburg, Germany. It is one of the most important Renaissance castles in northern Europe. It is the seat of the House of Schleswig-Holstein-Sonderburg-Glücksburg and was also used by the Danish kings. Situated on the Flensburg Fjord the castle is now a museum owned by a foundation, and is no longer inhabited by the ducal family. Its board of directors is chaired by Christoph, Prince of Schleswig-Holstein, the current titular duke and head of the House of Glücksburg and House of Oldenburg.
The castle was built from 1582 to 1587 by Nikolaus Karie for John II, Duke of Schleswig-Holstein-Sonderburg, (1545-1622) at the site of a former monastery, the building material was partly reused in the castle. The grounds of the monastery were then flooded to create a large pond almost entirely surrounding the castle.
The castle is built on a 2.5 metres (8.2 ft) high granite foundation that emerges from the water. The bricks used for the construction were mainly taken from the demolished monastery. The base area is a square with sides of nearly 30 metres (98 ft), consisting of three separate houses with their own roofs. While the great halls and the vestibule are situated in the middle house, the living space is in the two side houses. The chapel is the only room that is part of two houses.
On each corner there is a tower with a diameter of seven meters (22 feet). On the court yard front there are two stair towers which form the only connection between the floors.