The small rotunda church, tucked between the Sheraton and the Presidency, is believed to be the oldest building in Sofia.
Most historians agree that it was built in the early 4th century AD as a marthyrium: a small, typically round structure designed to hold important Christians relics.
Around this time, the building was used by the Roman Emperor Constantine the Great, who briefly held his court and palace across from the rotunda.
Constantine united the fractured Roman Empire, stopped the persecution of Christians and made Christianity the official religion in the Roman world. Subsequently, he decided to move the capital away from Rome and its pagan traditions.
We now know that Constantine settled in the city that would become Constantinople and later Istanbul. According to some sources, however, Constantine initially considered Sofia (then called Upia Serdica) among several other candidates as a potential site.
During the days of early Christianity, the building was used for mass baptisms, as the new religion was gaining large numbers of converts.
It went on to serve as a church until the 16th century, when it was transformed into a mosque by the new Ottoman rulers of the city.
Like many of Sofia's mosques, it was abandoned some 300 years later in 1858, after a strong earthquake destroyed its minaret.
In the early 20th century, restoration works began and the rotunda was stripped of its minarets and floral interior. Fragments of the 10th century frescoes were uncovered. If you find yourself here, look for the famous head of an angel!
The church was part of a larger complex of public buildings. Constantine’s Palace and one of the city’s major streets can be seen in the ruins excavated next to the rotunda.