Commissioned by Czar Alexander I in the 19th century to commemorate victory over Napoleon, St. Isaac's Cathedral (cathedral.ru) is a defining feature of St. Petersburg's skyline. Though it contrasts starkly with some of the more colorful Russian Orthodox churches, St. Isaac's is known for its enormous gold-gilded dome and ranks third in size in the world among domed cathedrals, according to Fodor's. The inside features frescoes, marble floors and granite columns.