St. Isaac's Cathedral is one of the world's largest cathedrals, was designed in 1818 by A. Montferrand.
The construction of the colossal building was a major engineering feat and it was opened in 1858.
The cathedral is graced with 112 solid granite columns weighing up to 114 tons each, and about 400 reliefs and bronze sculptures.
The interior of the cathedral is richly decorated with paintings, mosaics, gold, malachite, lazuli and marble.
The observation platform on the colonnade provides a magnificent view of the city.