Long before the castle there was Rude Abbey. Monks displaced from a dissolved double monastery at Schleswig settled first at Guldholm on the Langsee, then between 1209 and 1210 founded a Cistercian house here, beside an older motte-and-bailey castle whose remains still survive. They worked the estates around it for three centuries. The Reformation ended that: the abbey was secularised in 1538 and passed to the Danish king Christian III in 1544, its buildings reduced to an administrator's residence. Then Karies demolished it. The stone went into the new walls, the ground went under water, and the abbey survived only as a rumour until the pond was drained for facade repairs in 1962, 1969 and again in 2005. Each time, archaeologists went in. They found the foundations of the monastery church and a hypocaust heating system, and some of what they lifted out is displayed in the castle above it.

The man who ordered all this was Duke John, known to everyone as Hans the Younger, brother of King Frederick II of Denmark. The estates refused him homage, leaving him a duke without much of a duchy, so he built and traded instead - a successful early mercantilist who scattered castles across Schleswig-Holstein. His motto was Gott gebe Glück mit Frieden, God grant luck with peace, and the five initials G G G M F were set above the main entrance. The castle took its name from the middle word. Glück is luck; Glücksburg is Luck Castle. The plan is a square of almost thirty metres a side: three separate houses under three roofs, the great halls in the middle one, living rooms to either side. Around three thousand square metres of floor hold two halls, a vestibule, a chapel, and thirty-five rooms.

Hans died in 1622 and his son Philipp took Glücksburg, founding the elder ducal line. It ran over 150 years and produced no history to speak of - a family seat with a household of around eighty, not a court. The line died out in 1779, the fief reverted to the Danish crown, and the last duchess kept the place as her widow's residence until 1824. Then in 1825 King Frederik VI handed the castle and title to his brother-in-law Friedrich Wilhelm, who founded the younger line. Among Friedrich Wilhelm's ten children was the future Christian IX of Denmark, and through him the small castle on the Flensburg Firth became what people began calling the cradle of Europe. His daughter Alexandra married Edward VII, Dagmar married Alexander III of Russia, Thyra married the Crown Prince of Hanover; his second son became George I of Greece and a grandson became Haakon VII of Norway. Christian was known as the father-in-law of Europe, and the Danish, Norwegian and formerly Greek and Spanish thrones still carry his family.

You enter across a bridge into the basement, and the first room is the vestibule called the Green Hall - sparely furnished, but floored in Öland limestone in which fossils show through the tread of four hundred years. The chapel is the strangest room in the building - the only one that spans two of the three houses and the only one more than a storey high. It keeps a pulpit altar and font by Claus Gabriel from the mid-17th century, was rebuilt in Baroque style around 1717, gained a Marcussen organ in 1847, and gave up frescoes from the castle's first years during restoration work in 1973. To the west lies the family crypt, where thirty-eight members of the house are buried and the last interment took place in 1811. Upstairs, the Red Hall runs thirty metres long, ten wide and four high, named for wallpaper long gone, its vault carrying some of the earliest stucco work in Schleswig-Holstein and its walls paintings by Antoine Pesne.

War reached Glücksburg twice. In 1864, during the Second Schleswig War, it quartered Prince Charles of Prussia and then served as hospital and barracks; the personal union between the Danish crown and the duchies ended, and the castle passed into Prussian hands until Wilhelm I returned it to the family by decree in 1869. Eight decades later it held 200 Wehrmacht men as prisoners until the summer of 1945. On 23 May 1945 British troops occupied the castle as part of Operation Blackout, the round-up that ended Dönitz's Flensburg government; Albert Speer was arrested here. The family were held at gunpoint while the castle was looted and thirty-two coffins in the crypt were forced open. Many of the missing valuables came back later, after an appeal to Queen Mary. The family had already placed the castle in a foundation in 1922 to preserve it and open it to the public, and at Pentecost 1948 the museum opened again.