Haamstede Castle (Slot Haamstede) was built originally in the 12th century and it probably consisted of a wooden keep on a motte, circled by a moat. There is also archeological evidence of Roman habitation on this site. Around 1200 this castle came into the possession of Floris IV, Count of Holland. In 1229 the castle went to the Lords of Zierikzee through an exchange with Floris IV. The new inhabitants of the castle called themselves Van Haamstede.
The castle was destroyed by fire in 1525. Only the keep, built in the 13th century, survived. The other castle was rebuilt in the 17th century. The keep was provided with 2 square towers on both sides. The smallest one served as a stair tower. During the 19th and 20th century the castle was renovated twice until it got its present appearance.
At present Haamstede Castle is owned by the Vereniging Natuurmonumenten, a society for the preservation of nature monuments in the Netherlands.
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.