Thessaloniki ,Tomb of Galerius
About 200 m north of the Arch of Galerius is the Tomb of Galerius, now the Greek Orthodox Church of Agios Giorgios, better known as the Church of the Rotonda (or simply The Rotonda). The cylindrical structure was built in 306 on the orders of Galerius, who intended it to be his mausoleum. After Galerius's death in 311 (he was buried at Gamzigrad/Felix Romuliana near Zajecar), however, it stood empty until the Emperor Constantine I ordered it converted into a Christian church.
The Rotonda has a diameter of 24.5 m. Its walls are more than 6 m thick, which is one reason 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.
The building was a church for over 1,200 years until the city fell to the Ottomans. In 1590 it was then converted into a mosque, the Mosque of Suleyman Hortaji Effendi, and a minaret was added to the structure. It remained a mosque until 1912, when the Greeks captured the city during the Balkan War. It was then formally reconverted to a church, but the minaret was retained. The structure was damaged during an earthquake in 1978 but was subsequently restored. As of 2004, the minaret was still being stabilised with scaffolding. The building is now a historical monument under the Ephorate of Byzantine Antiquities of the Greek Ministry of Culture though the Greek Orthodox Church tries to reclaim it back for Orthodoxy every so often.
The Rotonda is the oldest of Thessaloniki's churches, and some publications in Greece claim that it is the oldest Christian church in the world, although there are a number of other claimants to that title. It is certainly the most important surviving example of a church from the early Christian period of the Greek-speaking part of the Roman Empire.