Turku Castle (, ) is a medieval building in the city of Turku in Finland. Together with Turku Cathedral, the castle is one of the oldest buildings still in use and the largest surviving medieval building in Finland. It was founded in the late 13th century and stands on the banks of the Aura River. The castle served as a bastion and administrative centre in Eastland, as Finland was known during its time as a province of Sweden. Only once did the castle figure in the defence of the realm, when Russian invaders from Novgorod destroyed Turku in 1318. It more frequently played a role in internal struggles for power within Sweden and the Kalmar Union. The castle's heyday was in the mid-16th century during the reign of Duke John of Finland and Catherine Jagellon. That was when the Renaissance Floor and King's and Queen's hall were built, along with other features. It lost its status as an administrative centre in the 17th century after Per Brahe's period as governor-general of Finland came to an end. Turku castle is today Finland's most visited museum, with attendance reaching 200,000 in some years. In addition, many of the larger rooms are used for municipal functions. History 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. The castle was ruled in different stages of its history by the castle sheriff, commander, regent, duke or governor-general. Its significance as a defensive fortress and administrative centre varied throughout the ages according to the political situation. Only once did the castle actually figure in the defence of the realm. This was when Russian invaders from Novgorod destroyed Turku in 1318. On the other hand, it frequently played a role