A start was made on building the castle in about 1280. The Swedish conquerors of Finland intended it originally as a military fortress. During the next two centuries its defences were strengthened and living quarters were added. The castle served as a bastion and administrative centre in Eastland, as Finland was known during the time as a province of Sweden. The main part of the castle was extended considerably during the 16th century after Gustav Vasa had ascended the Swedish throne and his son John headed the Finnish administration following his promotion to duke. The bailey was also supplemented and the round tower at the southeast corner of the bailey was added. Since then no part of the castle has been added or extended, only repaired.