Suomenlinna (Swedish: Sveaborg) is a sea fortress, which was built gradually from 1748 onwards by Sweden on six islands located at the entrance of Helsinki's harbour in Finland. The fortress consists of 200 buildings and 6 km of defensive walls. The work was supervised by the Swedish Admiral and Field Marshal Augustin Ehrensvärd (1710-1772).
en.wikipedia.org/wiki/Augustin_Ehrensv%C3%A4rd
The fortress is an especially interesting example of European military architecture of the time.
The fortress has served to defend three different sovereign states over the years: the Kingdom of Sweden, the Russian Empire and most recently the Republic of Finland.
The fortress is an Unesco World Heritage: "Fortress of Suomenlinna", dossier 583.
whc.unesco.org/en/list
