Alexander Nevsky Cathedral. This is one of the largest Eastern Orthodox churches in the world. It supposedly can hold 10,000 people, though since worshippers generally stand in Orthodox services, the number can be misleading.
The cathedral honors the Russian soldiers who died during the Russo-Turkish War of 1877-1878 -- the war that removed Bulgaria from Ottoman rule.
Between 1916 and 1920, the place was renamed the Sts. Cyril and Methodius Cathedral since Russia and Bulgaria opposed each other during World War I.