Saint Isaac's Cathedral or Isaakievskiy Sobor (Russian: Исаа́киевский Собо́р) in Saint Petersburg, Russia, is the largest Russian Orthodox cathedral in the city and was the tallest Eastern Orthodox church upon its completion. It is dedicated to Saint Isaac of Dalmatia, a patron saint of Peter the Great who had been born on the feast day of that saint. Architect - Auguste de Montferrand. Groundbreaking - 1818. Completed - 1858.