 The plague of 1710 killed the greater part of the inhabitants of Helsinki.The construction of the naval fortress Sveaborg (in Finnish "Viapori", today also "Suomenlinna") in the 18th century helped improve Helsinki's status, but it was not until Russia defeated Sweden in the Finnish War and annexed Finland as the autonomous Grand Duchy of Finland in 1809 that the town began to develop into a substantial city.Russians besieged the Sveaborg fortress during the war, and about one quarter of the town was destroyed in an 1808 fire.