L'Islet is a complicated and unique variation of a Cist-in-Circle monument. It dates from the Chalcolithic (2500 - 1800 BC). A central capstoned cist with an antechamber is surrounded by a D shaped ring of stones. Attached to the outer ring are four smaller rings two of which contain further slab lined cists. No bones were found but the central chamber contained a biconical pot. It is probable that the outer cists were later additions to an earlier single Cist-in-Circle.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.