The Monastery of St. Michael on the Heiligenberg (Saints" Mountain), was a branch of the nearby Lorsch Abbey. The ruined complex that can be seen today was built beginning in 1023. Within the nave are traces of the Roman temple of Mercury. The monastery was abandoned in the 16th century.
The first mention of the monastery is in the Lorsch codex, from the 12th century, which dates the founding of the monastery to 870. No remains of an earlier monastery have been found, so this might have been merely an estimate of the founding.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.