Olavinlinna is the main reason to come to Savonlinna and one of the great medieval castles of Scandinavia. Founded in 1475 by Erik Axelsson Tott as a Swedish outpost against the Novgorodians, it sits on a small rocky island between two lakes — a natural moat that made it almost impossible to besiege. The granite walls and three round towers are still essentially the 15th-century structure.
The interior can be visited on guided tours that run year-round (in English in summer, on demand the rest of the year). The two surviving towers can be climbed for views over the lake, the chapel hosts daily summer concerts, and from early July to early August the inner courtyard becomes one of the most spectacular opera houses in Europe for the Savonlinna Opera Festival. Tickets for the festival should be booked months in advance.