St. Martin church is a splendid building with an exceptionally beautiful Flamboyant Gothic chancel. It also contains a major work by Renaissance sculptor Ligier Richier, his Pietà. The church has been a historic monument since 1846 and it contains numerous works of art that have also been listed as historic monuments in their own right. The church was built in the 12th-13th centuries and it has been enlarged later.
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.