Turku Castle is a medieval structure in the city of Turku, Finland. Together with Turku Cathedral, the castle is one of the oldest buildings still in use in Finland. It is also 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 center in the historical region of Finland until the early 19th century. It played a role in power struggles within Sweden and the Kalmar Union and stood sieges, with additional battles fought outside its walls. The castle was at its peak in the mid-16th century, during the reign of Duke John of Finland and Catherine Jagellon. It lost its status as an administrative center in the 17th century, after Per Brahe's period as governor-general of Finland came to an end. Turku Castle is Finland's most visited museum. Visitations reach well over 100,000 people annually. Some of the rooms in the castle are used for municipal functions.