The structure of the arsenal consists of two large stone-built halls with vaulting supported by repeated pointed arches. The vaulting rests on ten piers, originally there were twenty two, the missing twelve and the structure they supported having been lost to centuries of coastal erosion. The main function of the arsenal was the building, repair and storage of warships. Amalfitan war-galleys were among the largest to be found in the Mediterranean during the Early Middle Ages.
The building now contains architectural and sculptural remains, a row-barge used in the Historical Regatta, a number of models of ships and it also acts as a venue for visual art exhibitions. Starting from December 2010, the Ancient Arsenals of Amalfi host the Compass Museum on the premises of the two aisles of the building, which were spared by the Amalfi seaquake of 1343.
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.