Erstavik estate was first mentioned in 1356 when it was given to Bengt Larsson. It was acquired by Petersen family in the 18th century and his descendants still own the castle. In the Great Northern War 1719 Russian fleet persecuted also Erstavik and several farms were burned down. The current main building is designed by Jean Eric Rehn and it was build in the 1760s. The three-storey house includes an chapel and two wings. interior is well-preserved sample of Rococo and Gustavian style.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.