St. Isaac’s Cathedral (or Isaakievskiy Sobor) is the largest church in St. Petersburg, and its magnificent dome, which measures 85 feet (26 meters) in diameter and is plated with pure gold, is a must-see site. The church was built by Tsar Alexander I after the defeat of Napoleon in 1858 as a national memorial to the fallen and the victorious. The beautiful paintings and mosaics in the majestic interior of the church can be explored with or without a guide.