The Church of St. George is the oldest surviving building in Sofia. It is a red-brick domed rotunda room with a circular plan on a square base with semicircular niches in the corners. Since the 4th century it has been used for baptismal ceremonies.
 It is part of a larger archaeological complex. Behind the apse, there are ancient ruins: a section of a Roman street with preserved drainage, foundations of a large basilica, probably a public building, and some smaller buildings. It has been assumed that some of the meetings of the Council of Serdica in 343 were held here.
There are five layers of partially preserved frescoes on the walls: the oldest is a Roman-Byzantine with floral motifs from the 4th century. There are frescos with angels from the 10th century. Furthermore there is a frieze with prophets along with frescoes depicting various scenes including the ascension and the assumption dating to the 11th and 12th centuries.
During the Ottoman rule in the 16th century, the church became a mosque. In the middle of the 19th century, it was abandoned by the Ottomans. Not much later, it began to be used again as a Christian church.