St John the Baptist Church was probably built in the 1180s as the first rector was installed in 1189. It was refurbished by Jasper Tudor, Duke of Bedford in 1485–95 and again in 1825–27 by the rector Rev. Robert Knight who added the vestry and altered the stone pulpit. The building was restored by John Prichard and John Pollard Seddon in 1860–61 and an organ chamber was added in 1885. More restoration work in 1903 and 1927 included reflooring the nave, reroofing the porch and installing oak stalls in the chancel. A meeting room and vestry wing were added in 1993.
The plan of the building is a tower in the west, nave, south porch, chancel, north-eastern chapel and northern wing. The church is built of roughly coursed grey rubble with grey or yellow ashlar dressings and has a slate roof with stone apex finials. The large and defensive west tower has wide-angle buttresses at each corner and a saddleback roof with embattled and corbelled parapets only on the northern and southern sides. The stone-tiled coping is topped by a weathervane. The northern and southern faces of the tower have four storeys of small round or square openings, although one on the south face is trefoil headed. The eastern facade has large shouldered openings above a corbel table that probably supported a wooden platform. The western side of the tower has a face corbel, reputedly of Saint John the Baptist, above a louvred gable opening, a clock and a three-light window above the ornate western doorway.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.