The church that was originally built where the Kazan Cathedral now stands, was called the Church of the Nativity of the Mother of God.  It was built in the 1730's and held the icon that symbolized the protection of the Royal family of the Romanoff's.  As this was such an important symbol, it was decided to build a cathedral worthy of housing the miracle working icon.  

Czar Paul I announced a competition amongst architects to design a cathedral worthy of housing the icon. Proposal after proposal was declined until plans were accepted from architect Andrey Voronikhin who modeled it after St. Peter's Cathedral in Rome.  It became the architects most famous work.  

The Kazan has the first metal dome created in history.  It is covered by tin and is decorated with pilasters.  

As your private tour guide in St. Petersburg, I'll bring you into the interior of the functioning cathedral.  It has three main halls that are over 60 meters high.  In 1932 the government in place turned the Kazan into a museum of religious history and atheism.  Thankfully, the museum was eventually closed, and Kazan was returned as a Cathedral.