It is also known (by its consecration and use) as the Greek Orthodox Church of Agios Georgios, and is informally called the Church of the Rotunda (or simply The Rotunda). The cylindrical structure was built in 306 AD on the orders of the tetrarch Galerius, who was thought to have intended it to be his mausoleum. It was more likely intended as a temple; it is not known to what god it would have been dedicated.
The Rotunda has a diameter of 24.5 m. Its walls are more than 6 m thick, which is why it has withstood Thessaloniki’s earthquakes. The walls are interrupted by eight rectangular bays, with the south bay forming the entrance. A flat brick dome, 30 m high at the peak, crowns the cylindrical structure. In its original design, the dome of the Rotunda had an oculus, as does the Pantheon in Rome.