When Peter the Great re-claimed the lands along the Neva River in 1703, he decided to build a fort to protect the area from possible attack by the Swedish army and navy. The fortress was founded on a small island in the Neva delta on May 27, 1703 (May 16 according to the old calendar) and that day became the birthday of the city of St Petersburg. The Swedes were defeated before the fortress was even completed. For that reason, from 1721 onwards the fortress housed part of the city's garrison and rather notoriously served as a high security political jail. Among the first inmates was Peter's own rebellious son Alexei. Later, the list of famous residents included Dostoyevsky, Gorkiy, Trotsky and Lenin's older brother, Alexander. Parts of the former jail are now open to the public...
In the middle of the fortress stands the impressive Peter and Paul Cathedral, the burial place of all the Russian Emperors and Empresses from Peter the Great to Alexander III. The Cathedral was the first church in the city to be built of stone (between 1712-33) and its design is curiously unusual for a Russian Orthodox church.
On top of the cathedrals’ gilded spire stands a magnificent golden angel holding a cross. This weathervane is one of the most prominent symbols of St Petersburg, and at 404 feet tall, the cathedral is the highest building in the city.
Other buildings in the fortress include the City History Museum and the Mint, one of only two places in Russia where coins and medals are minted. - in: http://www.saint-petersburg.com/virtual-tour/peter-paul-fortress.asp