Saint Panteleimon Monastery was built in 1843 and became monastery in 1987. According to inhabitants of Thassos, someone wanted to build it in favor of Saint Panteleimon. The workers started the building at a location, but the next day when they wanted to continue with the construction, the part they had built was found destroyed and their tools were missing. The same happened on the following days. One day they saw footprints on the ground and followed them until they found their tools nearby a natural spring. Finally, they built the monastery at that spot.
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.