The Rotunda of St. George is the oldest preserved building in Sofia, dating to the early 4th century CE. Originally built as Roman public baths during the reign of Emperor Galerius or Constantine the Great, it was later converted into a church. The red-brick cylindrical building features a 13.7-meter high dome and five layers of frescoes spanning from the 4th to 14th centuries. During Ottoman rule, it served as a mosque. The church is now an active Orthodox church and stands in an archaeological complex surrounded by the ruins of ancient Serdica, including Roman streets and building foundations.