St. Petersburg was founded by Peter the great in 1703 to protect the lands he had just conquered from Swedish forces.
During the course of the Northern war with Sweden, Russian forces defeated Swedish forces and conquered the lands in the Neva Delta. To protect this prize, Peter the great needed a fortress.
So at the spot where the Neva river meets the gulf of Finland off the Baltic sea, on the 16th - May 1703, tzar Peter the great ordered the city be built, beginning with a fortress.
Soldiers and peasants were ordered to build the fortress under the careful supervision of the tzar and his close associates. Due to the very damp climate and primitive conditions and long work hours, the workers died in great numbers but the work went on. By 1712, the city was big enough to become the new Russian capital.