During the 12th century and onwards, Osthoffen became a fortified castle. Due to a siege lead by the Bishop of Strasbourg in 1410, fire raged through the building, depriving it of its 3rd floor. It was rebuilt as a Rennaissance estate and a private dwelling. More changes were made in the 18th century: courtyard walls were removed and new wide windows enlightened both the façade and the interior. The castle lost its towers during the French Revolution by order of the new authorities, one of the first works undertaken by the new owner was to rebuilt them. Today Château d'Osthoffen is a hotel and event venue.
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.