The Church of St John the Baptist was founded by William FitzOsbern and given to an abbey in Normandy. The nave may be 11th century, as may be the Norman font. A north aisle was added at the end of the 12th century and a south aisle shortly afterwards. In the 14th century the chancel was rebuilt and the south porch was added. In the 15th century a chapel was added south of the chancel and east of the south aisle, and a four-centred arch was inserted in the south wall of the chancel to link it with the chapel. The north aisle was demolished, its arcade filled in and two-light Perpendicular Gothic windows inserted in each of the filled-in arches. The Perpendicular Gothic west tower was added towards the end of the 15th century. The square-headed windows in the south aisle were inserted in the 16th century and the spire was added to the tower probably early in the 17th century.
On the wall can be found a memorial portrait by John Flaxman, showing a woman holding pelicans in her hand in relief. The chancel has a modern reredos.
In front of the porch is a 19th-century Celtic cross by Joseph Clarke on the four steps of the old churchyard cross.
Near the Celtic cross a marble monument marks the grave of Edward Edwards, (1812–1886) the pioneer of the public library movement. The churchyard also contains four Commonwealth war graves of service personnel, three from World War I and one from World War II.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.