Sveaborg, the principal fortress of Finland, Russia, on the gulf of Finland, in the province and 3 m. S. E. of the town of Helsingfors, the approaches to which it defends, and under its municipal authority; pop. about 4,000. It is built upon seven granitic islands forming an ellipse, all of them strongly fortified, and connected either by causeways or bridges of boats. The principal fort is on Vargo island, on the south, and comprises a strong castle and barracks, and magazines excavated in the rock. The total number of cannon is 2,000, but it generally mounts 800; and the usual garrison of the fortress varies from 6,000 to 8,000 men, though the casemates have accommodations for 12,000. The harbor within, to which there is but one entrance, has room for 70 ships of the line. The fortress was erected between 1749 and 1758 by Count Ehrenswerd, field marshal of Sweden (who is buried within it), as a defence against Russia. In 1808 it was besieged by the Russians, and after two months the Swedish commandant and admiral Cronstedt, though amply supplied with the means of defence, capitulated (April 7). Left in the possession of the conquerors by the peace of Sept. 17, 1809, it was called the "'Gibraltar'of the North," and has since been regarded as the strongest fortress of Russia on the Baltic. In August, 1855, it was severely but unsuccessfully bombarded by the allied fleet.