St Isaac's Cathedral

The church itself is an architectural marvel. Built by the French-born architect Auguste Montferrand to be the main church of the Russian Empire, the cathedral was under construction for 40 years (1818-1858), and was decorated in the most elaborate way possible. When you enter the cathedral you pass through one of the porticos - note that the columns are made of single pieces of red granite and weight 80 tons (about 177,770 pounds) each. Inside the church many of the icons were created using moaic techniques and the iconostasis (the icon wall that separates the altar from the rest of the church) is decorated with 8 malachite and 2 lapis lazuli columns. The cathedral, which can accommodate 14,000 worshipers, now serves as a museum and services are held only on significant ecclesiastical holidays.

We saw priests enter the church and then disappear behind the altar. Not sure why they were there, 

But interesting to see them. The church can accommodate 1400 worshipers (no pews, everyone stands) and only has special service one or two times a year.nit is basically a museum today.