A visit to Helsinki wouldn't be complete without seeing Suomenlinna, Helsinki's iconic sea fortress that stretches over several separate islands. Originally named Sveaborg (Castle of the Swedes), this irregularly shaped bastion fortress was built in the 18th century to defend against invading forces and it has been controlled by three different sovereign states: Sweden, Russia, and currently Finland.

Today, Suomenlinna remains intact, inhabited, and is one of the most popular sights in Finland. Located just 1 km off the mainland, it only takes about 20 minutes by ferry to cross from Helsinki to Suomenlinna. This UNESCO World Heritage Site is worth checking out to see a distinct example of European military architecture of the time.