Situated on islands off Helsinki, Suomenlinna is an exclusive historical memorial and one of the biggest maritime fortifications in the world. Its history is strongly entangled with that of Finland and the Baltic region. Helsinki can also thank Suomenlinna for its early development and prosperity. Erected in the second half of the 18th century by Sweden on a group of islands situated at the doorway of Helsinki's harbor, this fort is an especially interesting paradigm of European military architecture of the time. One of the chief reasons for building Sveaborg was to help Sweden oppose the ambitions of Russia, whose chief armed base in the Gulf of Finland was Kronstadt, custom-built by Peter the Great to defend the conurbation of St Petersburg and as the home haven of a new Russian Navy to challenge Swedish marine power in the eastern reaches of the Baltic Sea.