Ancient rotunda in Thessaloniki, Greece, built in 306 CE by order of the Emperor Galerius, who was Caesar in the East, reigned by Diocletian. His domain was especially the Balkans.
It is suspected that the building was built as the planned mausoleum of Galerius, which was finally buried in Gamzigrad (present southern Serbia). After the death of Galerius in 311 CE the rotunda was empty and unused. It was not until the end of the 4th century CE that Theodosius I donated the rotunda to the Church.