Fortified with angular bastions around a central courtyard and supported by a broad embankment, this mighty fort complex was built between 1818 and 1831 by the Savoy Corps of Engineers. It was able to house 840 soldiers and 41,000 kilos of ammunition in its gunpowder magazine; it also had 1500 square meters of storehouses.
Renovation was carried out between the late 20th century and the early 21st century, subsidised by the European Union.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.