Schloss Leonberg was founded in 1248 by count Ulrich I of Württemberg. The original castle was modified between 1560 and 1565 by the master builder Aberlin Tretsch by order of the duke Christoph. The widow of Frederick I of Württemberg, Duchess Sibylla (1564-1614), used the castle from 1609 as a widow's residence. Later, the castle was still repeatedly used as a residence. The castle today is a tax office.
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.