Olavinlinna ("St. Olaf's Castle" or "Olofsborg") is one of the greatest medieval castles in Finland. It was built to secure the eastern border of the Kingdom of Sweden-Finland. The construction started in 1475 by Erik Axelsson Tott. Russians disturbed construction work sequently, because the castle was sited in Savonia to the Russian side of the border established by the Treaty of Nöteborg. Olavinlinna was completed probably in 1483 and there were first a main castle and three towers (Church Tower, Bell Tower and St. Erik's Towers).
Olavinlinna withstood several sieges by the Russians during the First and Second Russian-Swedish wars. Gustav Vasa ordered to erect fourth tower (a "Fat Tower") in 16th century and fifth ("Kilj Tower") was built in the 17th century. The castle was conquered first time by the invading Russians in Great Northern War on 28 July 1714. St. Erik's Tower was badly damaged in cannon fire and Russians demolished it. Olavinlinna was returned to Swedish in Treaty of Uusikaupunki, but they lost it constanly only 23 years later in the end of the Russo-Swedish War of 1741-1743.
Russians enhanced Olavinlinna fortifications and it withstood the siege of Swedish in the war of Gustav III in 1788. The Fat Tower was destroyed in an explosion of gunpowder supply in 1791. When Russians occupied Finland in 1809, Olavinlinna lost its defensive status. It was used as garrison, storage and prison and was abandoded in 1850s. Renovation was started in the end of the 19th century.
Currently, the castle hosts several small exhibitions, including the Castle Museum which displays artifacts found in the castle or related to it, and the Orthodox Museum which displays icons and other religious artifacts both from Finland and Russia. The castle forms a spectacular stage for the Savonlinna Opera Festival, held annually in the summer since 1912.
Olavinlinna is the initial model for Kropow Castle in the comic book King Ottokar's Sceptre, an album in the series of Adventures of Tintin created by Hergé.
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.