The Kajaani castle is considered to be the smallest stone-castle in Europe and the northernmost castle in the world.

The fixation in the title of Charles IX of Sweden is seen to be related to the construction of the Kajaani castle in 1604 CE, close to the border of Sweden-Finland with Russia.

The Kajaani castle (Finnish: Kajaanin linna, Swedish: Kajaneborg, Kajaneborgs slott, or with older spelling Cajanaborg) was built on the Ämmäkoski island of the Kajaani river in the centre of Kajaani, Finland, in the 17th century.