The Peter and Paul Fortress, on a small island in the Neva delta, was the original nucleus of St.Petersburg. Its construction was started on May 16th 1703. Originally earth ramparts reinforced by timber fortifications surrounded it, but in 1706 the Swiss architect Domenico Trezzini replaced these by a circuit of stone walls 8-13ft thick in the form of an irregular hexagon, with bastions at the corners named after the Tsar and five of his generals. The building of the fortifications was completed in the first half of the 18th century.
The fortress was never required for its original function, and from 1717 part of it was used as a political prison. The Cathedral of SS Peter and Paul, the centerpiece of the fortress, was built between 1712-1733. Domenico Trezzini as the burial church of the Tsars designed it. The Cathedral is a hall-church - a type rarely found in Russia. It is one of the great landmarks of the city, dominated by its tall tower and spire (400ft high). Peter the Great ordered that no building in the city should be higher. All the Tsars from Peter the Great to Nicholas the Second, with the exception of Peter the Second and Ivan the Sixth, are buried in the Cathedral.