Built by the Romans in the 4th century, the Church of St. George is the oldest building in Sofia. It's an Early Christian Rotunda, set among the remains of the ancient town of Serdica. The courtyard of the Sheraton Hotel is built around the well-preserved Rotunda. 

20th century restoration work on the interior has revealed three layers of medieval frescoes in the Rotunda, which had been hidden by plaster since the 16th century.  Historian and archeologists believe these frescoes date to the middle of the 10th century and portray 22 prophets.  The frescoes were painted over during the Ottoman period, when the building was used as a mosque.   

There are also the remains of a 2nd century street and other Byzantine ruins.  The Rotunda is now a museum and is under the protection of UNESCO.