In around 1300 Sostrup was an outlying farm known as Svorttorp but the first written record dates from 1388. The current castle was built in 1586 by councillor Jacob Seefeld. In 1807 the castle was bought by the Danish State and in 1829 by Jakob von Benzon. After this period some changes and restorations were made until the Danish State went into possession again in 1945. In 1946 it was run as a refugee camp. In 1960 the castle was bought by the Danish Cistercian Order. The castle was totally restored and today it is run as a conference centre.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.