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.
The old buildings are still in everyday use as housing, working space, maintenance facilities and visitor service facilities. There is a continuous building repair programme going on. The restoration of fortifications and old buildings requires considerable expertise. The 18th-century dry dock on Susisaari island is still used as a winter storage and repair facility for old wooden sailboats. There are dozens of historical underwater sites around the fortress.( http://www.suomenlinna.fi/en/fortress)