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.

The castle served as a defensive structure and administrative center in Österland, which was the name of the region known today as Finland, until the end of the 15th century.

[1] The main part of the castle was extended considerably during the 16th century, after Gustav Vasa had ascended to the Swedish throne.

His son, John III of Sweden, was the head of the administration and duke of Finland at that time.

In the short period between 1395 to 1398, Turku Castle may have been used as a base of operations by the Victual Brothers, a band of privateers turned pirates, invited there by the chief of the castle, Knut Bosson Grip [fi] due to some larger international political intrigue and allegiances concerning trade in the Baltic Sea.

In Paulus Juusten's episcopal chronicle, it is said that at the turn of the 15th century, pirates were causing havoc in and around the Turku archipelago.

[2] The castle was at its peak in the mid-16th century, during the reign of Duke John of Finland and Catherine Jagellon.

In the 18th century, the Renaissance bailey functioned as an administrative center, with the old main structure abandoned and in disrepair.

It was later handed back to the Finnish regional authorities, as the Grand Duchy of Finland was granted larger autonomy by the Tzar of Russia.

The city is currently entrusted with all Turku Castle related events and administration, on behalf of the government of Finland.

Entrance to the inner castle is not free, but they sell tickets at a lower price to pensioners, children, and the unemployed.