Saint Isaac's Cathedral or Isaakievskiy Sobor in Saint Petersburg is the largest orthodox basilica and the fourth largest cathedral in the world. 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.
The church on St Isaac's Square was ordered by Tsar Alexander I, to replace an earlier Rinaldiesque structure, and was the fourth consecutive church standing at this place.
The neoclassical exterior expresses the traditional Russian-Byzantine formula of a Greek-cross ground plan with a large central dome and four subsidiary domes. It is similar to Andrea Palladio's Villa La Rotonda, with a full dome on a high drum substituted for the Villa's low central saucer dome. The design of the cathedral in general and the dome in particular later influenced the design of the Lutheran Cathedral in Helsinki.
The exterior is faced with gray and pink stone, and features a total of 112 red granite columns with Corinthian capitals, each hewn and erected as a single block: 48 at ground level, 24 on the rotunda of the uppermost dome, 8 on each of four side domes, and 2 framing each of four windows. The rotunda is encircled by a walkway accessible to tourists. 24 statues stand on the roof, and another 24 on top of the rotunda.
The cathedral's main dome rises 101.5 metres (333 ft) and is plated with pure gold. The dome is decorated with twelve statues of angels by Josef Hermann. These angels were likely the first large sculptures produced by the then novel process of electrotyping, which was an alternative to traditional bronze casting of sculptures. Montferrand's design of the dome is based on a supporting cast iron structure. It was the third historical instance of cast iron cupola after the Leaning Tower of Nevyansk (1732) and Mainz Cathedral (1826).