Kazan Cathedral is a striking neo classical edifice set in the middle of Nevsky prospekt. It's dome is visible from a distance, and while you approach it you'll see colonnades around it in both directions. It strongly reminds on Rome's St. Peter, and is in fact built according to it in 1810.
Apart from the interiors beauty, specially the altar, most interesting I found was the Gen. Kutuzov tomb inside.
Our Lady of Kazan was general's patron saint who he asked for help when Napoleon invaded Russia in 1812. After defeating him, Kutuzov was buried in the Kazan Cathedral in 1813.