The world's most northern in the context of "St. Olaf's Castle"

Olavinlinna (Finnish: [ˈolɑʋinˌlinːɑ]; Swedish: Olofsborg), also known as St. Olaf's Castle, is a 15th-century three-tower castle located in Savonlinna, Finland. It is built on an island in the Kyrönsalmi strait that connects the lakes Haukivesi and Pihlajavesi. It is the northernmost medieval stone fortress still standing and also the best preserved of all the medieval castles in the Swedish realm.The castle forms a spectacular stage for the Savonlinna Opera Festival, which was held for the first time in the summer of 1912.