Tiny Zayachy Island contains the oldest building in town - the Peter & Paul Fortress. It was built in 1703 while Peter the Great was still roughing it in a log cabin overlooking the embryonic city, and designed according to plans by the man himself.

Its original purpose was to defend the land newly acquired from the Swedes. However, its main use up to 1917 was as a political prison and the first inmate was Peter's own son Alexey, who was followed by other notables such as Dostoevsky, Gorky, Trotsky and Lenin's older brother, Alexander. The cathedral, though plain on the outside, has a magnificent baroque interior. Most of Russia's Romanov rulers are buried here.