Sønderborg Castle Sønderborg Castle ( ) is located in the town of Sønderborg , Denmark on the island of Als in South Jutland . It houses a museum focusing on the history and culture of the area . The castle is located in the middle of the town , in a park setting overlooking Als Fjord . The museum is open year round . History . Sønderborg Castle began probably as a fortified tower constructed by Valdemar the Great in 1158 , built on an islet in Als Strait ( " Als Sund " ) that later was connected to the island of Als . The castle was built to provide protection against attacks by the Wends and was part of a larger system of fortifications . Over the centuries , the castle has gradually been enlarged and rebuilt . In the years following construction of Valdemar ' s fortified tower , an important struggle developed between the Danish king and the duke of Schloneswig over ownership of the island of Als and the town of Sønderborg . Ownership of the castle changed hands many times . A peak in the history of the castle was the wedding of Valdemar IV ( Valdemar Atterdag ) ( ca . 1320 - 1375 ) to the duke ' s sister , Helvig of Schleswig . Around 1350 , the castle was expanded significantly by the addition of both the Blue Tower ( " Blåtårn " ) and huge outer walls . In 1490 , the fortress became the property of the Danish crown . Both King Hans and his son Christian II extended Sønderborg Castle and made it into one of the country ' s strongest fortresses . In 1532 , Christian II was lured into an ambush and taken to Sønderborg Castle , where he was held as a prisoner of state for seventeen years . Legend tells that the dethroned king was confined to the Blue Tower , but in reality he lived in comparatively lavish circumstances and probably was allowed freedom of movement within the outer walls , if sometimes under closely guarded conditions . The former king sometimes even joined the noble hunts at Als . The false story about the restless king walking around a round table making a groove in it with his thumb was fabricated long afterward , during the seventeenth century . Christian III in the mid - sixteenth century had the fortress modified and converted into a four - wing castle by architect Hercules von Oberberg between 1549 - 57 . King Hans ' s west wing was preserved and a further three wings were added in the new Renaissance style . After Christian III ' s death in 1559 , Hercules von Oberberg built the unique castle chapel in 1568 - 1570 for Queen Mother Dorothea . After Dorothea ' s death in 1571 , the castle passed into the ownership of Hans II , Duke of Schleswig - Holstein - Sonderburg - Plön ( also known as Hans the Younger ) . Under his rule , the castle became the center of a tiny duchy , Schleswig - Holstein - Sonderburg . This , however , was divided after his death in 1622 . The castle remained in the hands of the dukes of Southern Jutland until 1667 , when the ruined duchy of Sønderborg was attached to the Danish throne and the castle became a Danish estate . The duchy ’ s representative , the prefect ( ‘ ’ Amtmanden ’ ’ ) , took up residence at the castle . It was otherwise more or less unused in the years 1667 - 1718 . In 1718 - 1726 , Frederik IV had the castle rebuilt in Baroque style by contractor general Wilhelm von Platen . The Blue Tower was demolished in 1755 , and in 1764 the castle passed into the hands of the Duke of Augustenborg ; but , contrary to expectations , the castle did not become the duke ' s residence . Instead , it was rented out as a warehouse . During both the first and the second Slesvig wars ( 1848 - 1850 , 1864 ) , Sønderborg Castle was used as a camp hospital and for quartering Danish troops . After the war of 1864 , the province and the castle became Prussia n property and served as barracks from 1867 until the area was reunified with Denmark in 1920 . The last duke of Augustenborg , Ernst Günther , allowed Sønderborg County Museum to move into a part of the castle in 1920 . The next year , the Danish state bought the castle from the Duke , taking over the castle in 1921 and allowing several institutions to use it as long as they paid heed to the expanding museum . In 1945 and 1946 , the castle was used as an internment camp for persons charged with offenses to the state . The Royal Inspectors of Listed State Buildings , Peter Koch and Jørgen Stærmose , conducted a thorough restoration of the castle from 1964 to 1973 , returning it to the Baroque form it had been given by Frederik IV in the 1720s . The windows from the barracks era were even replaced with " masks " , windows with broad wooden frames made of planks like the ones in Platen ' s castle . The museum and grounds . Since 1921 Sønderborg Castle has been the home for The Sønderborg Castle Museum ( " Museet på Sønderborg Slot " ) , which is the main museum for the former Duchy of Slesvig . The museum houses local and regional history collections from the Middle Ages to the present day , but with especial focus on the Schleswig wars of 1848 - 50 and 1864 , World War II , and the Reunification of 1920 . The museum also hosts exhibitions on navigation , textiles and handicrafts , and has a small art collection with works by prominent Southern Jutland painters over the years . The original ramparts around the castle became a visible part of the gardens in the 1970s . The Chapel of Sønderborg Castle . The unique chapel of Sønderborg Castle , also known as Queen Dorothea ' s Chapel , was built 1568 - 70 by Hercules von Oberberg for Queen Mother Dorothea reflects the changing times in Denmark after the Reformation . It is almost untouched , and is considered to be one of Europe ' s oldest and best - preserved Lutheran royal chapels . Many of the items in the chapel were created in Antwerp at painter Frans Floris ’ workshop . One of the royal couple ’ s sons , Duke Hans II , had a burial room built in the chapel with an imposing portal of marble and alabaster . The chapel ’ s organ is attributed to organ builder Hermann Raphaëlis , and it is estimated to have been built ca . 1570 . The paintwork on the organ case dates mainly from 1626 . Hermann Raphaëlis ( ca . 1515 - July 8 , 1583 ) was of Dutch descent and was the son of organ builder Gabriel Raphael Rottensteen . Raphaëlis was summoned to Denmark ca . 1550 , supposedly to build an organ for Roskilde Cathedral . Apart from the instrument at Roskilde Cathedral , built in 1555 , he also built an organ for the Chapel of Copenhagen Castle in 1557 . Raphaëlis later settled in Saxony , where he among other things built organs in the chapels of castles belonging to Elector August of Saxony , the Queen Dorothea ’ s son - in - law . The organ was reconstructed in 1996 by Danish organ builder and historian , Mads Kjersgaard , according to 16th century traditions with regard to tone and craftsmanship . The museum has a permanent exhibition of preserved parts from the Renaissance organ , including a rat ' s nest taken from one of the organ ’ s bellows . Under the west gallery of the chapel there is a choir loft over which there was originally a royal chair ; this disappeared when the organ was built in its present position in 1626 . The chairs were made by local wood carver , Niels Tagsen , who also fashioned the pulpit and the 3 figures which were placed on the organ in 1626 . The organ case and doors were partly repainted by Wulf Petersen of Sønderborg on the same occasion . The chapel may be visited during the opening hours of the museum . The Great Hall . The Great Hall ( ‘ ' Riddersal ’ ’ ) is 34 meters long and was the reception room for ducal guests , and the scene of parties and dances . The room is still used for special events , such as in connection with the marriage of Queen Margrethe II ' s niece , Princess Alexandra of Sayn - Wittgenstein - Berleburg to Count Jefferson - Friedrich von Pfeil und Klein - Ellguth in 1998 . Management . Sønderborg Castle is owned by the Danish state , run by the Palaces and Properties Agency ( " Slots - og Ejendomsstyrelsen " ) and used by Sønderborg Castle Museum , which is jointly owned by the municipality of Sønderborg and by South Jutland County .