The gold-domed Alexander Nevski Cathedral was built in the early 20th century in memory of the 200,000 Russian, Ukrainian, Belorussian and Bulgarian soldiers, who died in the Russo-Turkish War, 1877–1878. It is one of the largest Eastern Orthodox cathedrals in the world. The cathedral’s gold-plated dome is 53 m high, with the bell tower reaching 50.52 m.
The Alexander Nevski cathedral serves as the cathedral church of the Patriarch of Bulgaria and is one of the largest Eastern Orthodox cathedrals in the world, as well as one of Sofia’s symbols and primary tourist attractions. The St. Alexander Nevski Cathedral in Sofia occupies an area of 3,170 square metres (34,100 sq ft) and can hold 10,000 people inside. It is the second-largest cathedral located on the Balkan Peninsula, after the Cathedral of Saint Sava in Belgrade
The construction of the St. Alexander Nevski Cathedral started in 1882, when the foundation stone was laid, but most of it was built between 1904 and 1912.It is named after Saint Alexander Nevsky was the Prince of Novgorod, Grand Prince of Kiev and Grand Prince of Vladimir during some of the most trying times in the city’s history. He was proclaimed Saint of the Russian Orthodox Church in 1547.
The name of the cathedral was briefly changed to the Sts. Cyril and Methodius Cathedral between 1916 and 1920 (since Bulgaria and Russia belonged to opposing alliances in World War I), but then the initial name was restored. The St. Alexander Nevsky Cathedral was proclaimed a monument of culture on 12 September 1924
The Alexander Nevski cathedral was designed by Alexander Pomerantsev. The construction and decoration were done by a team of Bulgarian, Russian, Austro-Hungarian and other European artists. The marble parts were created in Munich, the metal elements for the gates in Berlin, while the gates themselves were manufactured in Karl Bamberg’s factory in Vienna, and the mosaics were shipped from Venice.
There is a museum of Bulgarian icons inside the cathedral crypt, part of the National Art Gallery. The church claims that the museum contains the largest collection of Orthodox icons in Europe.
There is an outdoor antiques/flea market on the same square as Alexander Nevski cathedral and Sveta Sofia church. All kinds of things can be found here, from badges and metal signs from the Soviet period to works of art.. There is also a separate area where old ladies sell lace and knitted socks.
You can read also about the Saint Sofia church.