Olofsborg

Olofsborg, known in Finnish as Olavinlinna, rises from a rocky island in the channel between Haukivesi and Pihlajavesi. Erik Axelsson Tott began the castle in 1475 to strengthen Sweden's eastern frontier. Water and a swift current made the site difficult to approach, while the lake routes brought building materials to its walls. Its round towers and high curtain walls formed an imposing late medieval stronghold in a sparsely settled region.

The builders first raised wooden defences against Russian attacks, then constructed a main castle with three towers and a two-towered outer bailey. The main castle took roughly a decade to become defensible, and the whole five-towered complex was finished by the end of the century. Russian forces attacked again during the war that began in 1495. The Treaty of Teusina in 1595 moved the border farther east, leaving the castle well inside Swedish territory. A settlement grew nearby and received a town charter in 1639 as Savonlinna, or Nyslott.

The Great Northern War brought the frontier struggle back to the castle. After a siege it surrendered to Russia in 1714, but the Treaty of Nystad returned it to Sweden in 1721. It passed to Russia again under the Treaty of Åbo in 1743. Russian engineers subsequently strengthened its defences with angular bastions, adapting the medieval fortress to the warfare of a later age.