Built directly on rock in the middle of the lake (now accessed via bridges), this heavily restored 15th-century fortification was constructed as a military base on the Swedes' restless eastern border. The currents in the surrounding water ensure that it remains unfrozen in winter, which prevented enemy attacks over ice. To visit the castle's upper levels, including the towers and chapel, you must join an hour-long guided tour. Guides bring the castle to life with vivid accounts of its history.
The stories you'll hear on the tour are fascinating. The soldiers, for instance, were partly paid in beer – 5L a day and 7L on Sundays, which makes the castle’s frequent changes of hands more understandable. English-language tours leave hourly June to September; check the website for language options at other times. You'll need to be reasonably fit, as the tour involves climbing up and down steep and narrow winding staircases. A joint ticket to visit the castle and nearby Riihisaari costs adult/child €12.50/5.50.