Until recently very little was known about the building history of Huis Hatert. The tower was probably built in the second half of the 14th century. The west and south sides of the building show signs of walls that were once attached to the tower, which makes it likely that the tower used to form a part of a greater complex and that it served as a gate tower located on one of the corners of a lager castle. Its relatively thin walls and small size also hint at this. In later times the tower underwent great changes. Floor heights were changed, openings were blocked and new wings were attached to the tower aft er the former walls had been torn down. It is evident that one of the cellars of Huis Hatert dates back to the 16th century and one of the wings to the 18th century. In the 19th century a new gable was placed in front of the 18th century wing and tower to create a more uniform appearance.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.