Kajaani Castle (Finnish: Kajaanin linna, Swedish: Kajaneborg, Kajaneborgs slott, older spelling Cajanaborg) was built on the Ämmäkoski island of the Kajaani River in the centre of Kajaani, Finland, in the 17th century. Today, only roofless ruins remain of the castle. The castle functioned as an administrative centre, prison, and military strongpoint. The most famous prisoner was the historian Johannes Messenius, who was forced to live in the poor conditions of the castle from 1616 to 1635. In 1917 a hydro-electric plant was built at Ämmäkoski.