Suomenlinna, until 1918 Viapori, (Finnish), or Sveaborg (Swedish), is an inhabited sea fortress built on six islands, today within Helsinki, the capital of Finland. It is a UNESCO World Heritage site and popular with both tourists and locals, who enjoy it as a picturesque picnic site. Originally named Sveaborg (Fortress of Svea), or Viapori as called by Finns, it was renamed Suomenlinna (Castle of Finland) in 1918 for patriotic and nationalist reasons, though it is still also known by its original name. It is an example of the Star Fort style of fortification.

Sweden started building the fortress in 1748 as a protection against Russian expansionism. The general responsibility for the fortification work was given to Augustin Ehrensvärd. The original plan of the bastion fortress was strongly influenced by the precepts of Vauban, the foremost military engineer of the time.

In addition to the island fortress itself, seafacing fortifications on the mainland would ensure that an enemy would not acquire a beach-head from which to stage attacks. The plan was also to stock munitions for the whole Finnish contingent of the Swedish Army and Royal Swedish Navy there. In the Finnish War the fortress surrendered to Russia on May 3 1808, paving the way for the occupation of Finland by Russian forces in 1809.

http://en.wikipedia.org/wiki/Sveaborg