The Church of St. George is an Early Christian red brick rotunda that is considered the oldest building in Sofia. Built by the Romans in the 4th century CE, it is mainly famous for the 12th-14th century frescoes inside the central dome. Three layers of frescoes have been discovered, the earliest dating back to the 10th century. Magnificent frescoes of 22 prophets over 2 metres tall crown the dome. Painted over during the Ottoman period, when the building was used as a mosque, these frescoes were only uncovered in the 20th century.