Valdemar's Castle was commissioned by King Christian IV (1588–1648) and built between 1639 and 1644 by his architect Hans van Stenwinkel.
The king’s plans for his new castle were that the house should become the home for his son Valdemar Christian, who was born to him by Kirstine Munk. King Christian was renowned for his interest in building. On the island of Tåsinge, belonging to his mother in law Ellen Marsvin, the king decided to build a castle for his young son. However, Valdemar Christian never moved in. He was killed in battle in Poland in 1656.
In 1678 the naval hero, Admiral Niels Juel, was given title to the castle and the land on Tåsinge after his victory over Sweden in the Battle of Køge Bay in 1677. The estate was transferred to him as payment for the Swedish ships captured in the battle.
The present owner, Baron Iuel-Brockdorff, who is 11th generation of the Juel family, took over his childhood home from his father in 1971 and lives in the castle with his wife and family. Valdemars Slot has been open to the public since 1974. The castle is open from May to October and on public holidays. The castle features a large chapel, a toy museum, the Iuel-Brockdorff family's big game trophy collection and a local maritime museum. As the castle lies near the beach, it is popular for visitors to come by ferry on the Helge from Svendborg.