Stunning!Beautiful architecture!You'll see some orthodox traditions,people go to light candles and pray.women don't have to cover their head to go in,but I think it's better to bring a scarf with you!
The cathedral is right in the centre and has free entrance, so there is no reason to skip a visit. You'll see some orthodox traditions, as people come here to light candles and pray to the icons.
If you are in the city you must visit Kazan. The cathedral is nothing exceptional but the oitside is worth the trip. Around Xmas decorations in front are very interesting!