The stone church of Grundsunda was probably built in the 14th century. Arches were added later in the Middle Ages as well as the porch and sacristy. Walls and the roof were decorated with murals around 1600, but them were overpainted in the 19th century. The wooden belfry was erected in 1794. The unique font dates from the 17th century and is made of single piece of wood. The pulpit was carved by Tomas Kiempe in the 1720s.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.