Barry Castle was a fortified manor house built on the site of an earlier Norman structure. It was raised by the powerful de Barry family, who played an important role in both the conquest of South Wales and Ireland. The structure was ruinous by the sixteenth century.
By the late 13th century the castle had two stone buildings on the east and west sides of a courtyard, but nothing now remains of these above ground. Early in the 14th century the castle was strengthened by the addition of a large hall and gatehouse on its south side, and the ruins of these can be seen today.
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.