The Espoo Cathedral is a medieval stone church built in the last half of 15th century. The church is thus the oldest preserved building in the city. The church was originally designed in by an unknown "Espoo master" and built between 1485 and 1490 under his supervision.
The only remaining parts of the medieval church are the eastern and western parts of the nave. The weapons room was removed between 1804 and 1806 and certain other parts of the church, including the original sacristy, were taken apart between 1821 and 1823 when the building was converted into a more spacious cruciform church.
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.