The Roman Rotunda is Thessaloniki’s most magnificent ancient monument. Built in the early fourth century, it was most likely intended to be Emperor Galerius’ mausoleum (although he was not buried here) and was part of the complex that included the Galerius Palace and the Arch of Galerius. Emperor Theodosius the Great, who was baptized as a Christian in Thessaloniki, converted the mausoleum into a Christian church in the late fourth century.