Suomenlinna is an old (300 ish years) fortress originally built by Sweden to stop the Russians. It’s a beautiful island that is now rented as apartments.
Sweden built it to stop the Russians. Finland fought a war with Sweden to become independent. The Russians took the fortress by trickery. When the Russians had one of their revolutions in about 1917, the Finns took the island back.
My favorite story of the island is when they used it as a decoy to save Helsinki from Russian bombs in the second world war. They blacked out Helsinki, put lots of lights on the island and 95% of the bombs fell short of the city.