St. Isaac's Cathedral (Cathedral of St. Isaac of Dalmatia) - the largest and most famous church in St. Petersburg, which is the dominant and second tallest building, after. Peter and Paul Cathedral, in the center of St. Petersburg. St. Isaac's Cathedral is one of the greatest and largest domed structures in the world, second only in size to the cathedrals of St. Peter in Rome, St. Paul in London and Santa Maria del Fiore in Florence.