We visited the "Gibraltar of the North", Suomenlinna. The Swedes built it in the 19th century to guard against a Russian invasion by sea. It was to control the entrance to the Finnish wilderness -- important because Finland was a buffer between Russia and Sweden.
The fortress spans four islands off the Finnish coast and did help the Swedes hold off Catherine the Great. The "dry dock" helped the Swedes maintain a standing navy of shallow-water "archipelago" ships in the Gulf of Finland and launch them once the ice thawed.
Catherine the Great's successor, Tsar Alexander II, invaded Finland by land and the fortress was of no help at all. So, the fort passed into Russian control. The fortress was not of much help to the Russians either. British and French ships were able to ply the Gulf of Finland with impunity because the fortress's cannons didn't have the needed range. The Russians enhanced the fortress with bigger cannons. They also used camouflaged gunnery areas, sort of like the Hobbit's homes in Lord of the Rings.
When the Finns declared independence, the fort passed into Finnish control. So, the fort (constructed with great cost over 40 years) was defended successfully exactly once. In any case, the Finns hated the Russians so much that they lopped off the cupolas from the Orthodox church that graced the island and put a lighthouse on top of the church tower.
The Swedish count who conceived and built Suomenlinna is buried here. In his day, it was called Sveaborg ("castle of the Swedes"). But because Finns can't pronounce the letter 'b', the name got corrupted to "Viapori". The Russians used the name "Viapori" after they captured it. When the Finns declared independence from Russia in the early 1900s, they called the fortress "Suomenlinna" (castle of the Finns). But here's the clincher. About 5% of Finns speak Swedish, so it's an official language here (think of the French in Quebec). So, the fortress is called "Sveaborg" in Swedish and "Suomenlinna" in Finnish and both are official names. The municipal ferry that took us to the island had both names on it.