Suomenlinna, Helsinki, Finland. Situated on a group of islands off Helsinki, Suomenlinna was built during the Swedish era as a maritime fortress and a base for the Archipelago Fleet. Work on the fortress was begun in the mid-18th century. Today, it is a UNESCO World Heritage Site and one of Finland’s most popular tourist attractions. Suomenlinna is also a district of the city of Helsinki, with a permanent population of more than 800.