The Church of St. George (Rodtonda Sveti Georgi) is said to be the oldest building in Sofia. It's history dates back to the 4th century, being constructed by the Romans. It is mainly famous for the 12th and 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 mosque, these frescoes were uncovered in the 20th century.