the later Danish king Christian IX. – the progenitor of today's Glücksburg line on the Danish throne. The Danish royal family often used their relatives' castle as a summer residence. From 1854, King Frederick VII occasionally resided at Glücksburg, until he died childless here in 1863. According to the London Protocol of 1852, Christian IX. of the Glücksburg line became his successor. Under him, the castle gained a reputation as the cradle of Europe, and the new king was often referred to as Europe's father-in-law. From Christian's marriage to Princess Louise of Hesse, three daughters were married into the royal houses of England and Russia: Alexandra married the later Edward VII, Dagmar the later Tsar Alexander III and the youngest daughter Thyra the Duke of Cumberland. The second son became King George I of Greece and the grandson Carl became King of Norway. From this time forward, the House of Glücksburg is related to almost all major European dynasties. In the possession of Wilhelm I During the Second Schleswig War in 1864, the castle served as quarters for Charles von Prussia and was later even used as a hospital and barracks. With the war, the long personal union of the Danish royal family and the Schleswig-Holstein duchies ended, and the castle passed into Prussian ownership. On September 16, 1868, the Prussian King Wilhelm I visited the castle during a visit to Flensburg to decide on its future fate and use. The Flensburger Nachrichten newspaper reported on September 22: "[...] Glücksburg had festively adorned itself for the arrival of the king; There were three gates of honor, one at Ruhethal, the second in front of the entrance to the town, the third at the driveway to the castle, and flags and floral decorations in abundance. The king arrived the 16th, at 3 p.m., at the second gate of honor, Pastor Vogel gave a brief address, which the monarch graciously returned. A few peasant girls had gathered at the castle to offer the father of the country butter, bread, cheese and fruit as well as fishing products. [...] The whole stay in the village and at the castle lasted only a small hour. " King Wilhelm I showed no further personal interest in the property and the War Ministry no longer needed it. In 1869, King Wilhelm I transferred the castle back to the ducal family by means of a "highest decree".