Saint Isaac's Cathedral or Isaakievskiy Sobor in Saint Petersburg, Russia is the largest cathedral in the city and was the largest church in Russia when it was built (101.5 meters high). It was 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. The cathedral took 40 years to construct, under Montferrand's direction, from 1818 to 1858.
The severe neoclassical exterior expresses a traditional Russian-Byzantine formula: a Greek-cross groundplan 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.
Saint Isaac's Cathedral Wikipedia Entry