This triangular fortress with three towers on one side is dedicated to St Olaf. It was built in 1475 by Erik Axelsson Tott to protect the kingdom's eastern provinces. It was extended, and two towers added in the 16C-17C, but these were razed when the Russians took possession of the site (18C); it also served as a prison for a time. From the towers (30m), there are views of the fabulous landscape of lakes and islands. The permanent exhibition charts the history of the castle and everyday life within it walls.