Olavinlinna, also known as St. Olaf's Castle, is a 15th-century fortress located in Savonlinna, Finland. Built on an island between two lakes, it is the northernmost medieval stone castle still standing. The castle was founded in 1475 by Erik Axelsson Tott to secure the border and withstand Russian attacks. Despite facing sieges and fires over the centuries, Olavinlinna remains a popular tourist attraction and hosts the annual Savonlinna Opera Festival, making it a must-visit destination for history buffs and opera enthusiasts alike.