The Kajaani Castle Ruins, located in Kajaani, Finland, were built in the early 17th century on an island in the Kajaani River. Constructed by King Charles IX of Sweden to protect the region of Kainuu, the castle saw various uses over the years, from being a prison to a noble residence. However, during the Great Northern War, Russian forces destroyed the castle with explosives in 1719. Today, only its roofless ruins remain, serving as a reminder of its once strategic significance and turbulent history.