During the course of the Northern War with Sweden, Russia's forces gradually moved from Lake Ladoga down the Neva River to the Swedish fort of Nienchanz. After an 8-day siege on May 1 1703, the Swedish garrison surrendered. To protect the newly conquered lands on the Neva delta Peter the Great needed a fortress, but Nienchanz was small and badly damaged. Looking for a site for his new fortress Peter the Great chose the Island of Enisaari (Hare's Island), which was known to the Russians as Zayachii ostrov. On May 16 1703 (May, 27 by the modern calendar) St. Petersburg's fortress (the Peter and Paul Fortress) was founded and that day became the official birthday of the city. Several days later a wooden Cabin of Peter the Great was built, and became the first residential building in the new city.
The original clay walls and bastions of the fortress were completed by the end of summer 1703 under the careful supervision of the Tsar and his close associates. The builders of the fortress (mostly soldiers and peasants) worked in very primitive conditions, since the climate was very damp, good housing nonexistent and food in very short supply. Working from dawn to dusk, they died in great numbers, but the war still went on and the fort had to be completed as soon as possible.
By August 1703 the new settlers in Peter's city had already encountered the infamous St. Petersburg floods. Due to the boggy nature of the terrain, the area was considered unhealthy for a town, but it had tremendous strategic importance, so Peter the Great continued constructing the city despite all the losses and extra expenditures. For its first few years the St. Petersburg of Peter the Great was limited to a small town around the fortress, but by 1712 it had grown enough to become the new Russian capital.