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 then known, during the Swedish period.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 in internal struggles for power within Sweden-Finland and the Kalmar Union. Not until the end of 16th century did it really enjoy peace. The castle 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.
Many accidents have assailed the castle, especially numerous sieges. In 1614, when King Gustav II Adolf visited the castle, a tremendous fire destroyed the wooden structure of the main castle almost completely. After this the main castle was abandoned and used partly as a store, partly just stood empty. A new accident beset the castle in the summer of 1941 soon after the Continuation War had begun when an incendiary bomb hit the main castle.
The bailey functioned as an administrative centre when the main castle stood empty. In addition to the governor-general the lord-lieutenant lived there during the 17th century and after the period of Greater Wrath it housed the provincial government for some time. One of the most colourful periods in the castle's history was when it was a prison from the 18th century to the end of 19th century. When the Finnish war began in 1808 the castle was used by the Russian navy and only handed over to the Finnish authorities after the country had been granted autonomous status within the Russian empire a couple of decades later. The Turku Historical Museum founded in 1881 began its occupation of the bailey immediately after the museum was founded and later it was given quarters in the castle itself.
The renovation of the castle, which was begun before the Second World War and interrupted by Finland's two wars with the Soviet Union, was completed in 1987. The castle was handed over to its users completely restored on 12 October 1993. The building is owned and maintained by the Finnish state and is entrusted to the use of the city of Turku. The castle functions in its entirety as a historical museum as part of the Turku provincial museum. In the main part of the castle are banquet rooms for the city of Turku, a church for the local congregation and also restaurants in both the main castle and the bailey. Today the castle is once more experiencing a heyday as it has achieved the status of being one of Finland's most visited museums.
Text is from Wikipedia.