Helsinki is the European capital containing Suomenlinna, a sea fortress founded in 1748 on islands off its coast.
Sweden began constructing the fortress in 1748, when Finland was part of the Swedish realm and Sweden sought stronger defenses against Russia. The site was originally called Sveaborg, meaning Sveaborg fortress, and it was built across several islands at the entrance to Helsinki's harbor.
The fortress surrendered to Russian forces during the Finnish War in 1808. Russia then controlled the site for more than a century, and it became an important military base. After Finland became independent in 1917, the fortress was renamed Suomenlinna, meaning Castle of Finland.
Suomenlinna is now a UNESCO World Heritage Site and a living district with museums, residences, services, and a naval academy. It is reached by ferry from Helsinki's Market Square. The fortress is not located in Stockholm's archipelago, despite its Swedish origins.