The Rotónda is part of the Galerius complex (Anáktora Galeríou), which was the ancient political centre of Thessaloníki under Galerius (300 AD). The building was erected in the 4C AD as a circular mausoleum for the Emperor Galerius. In the following century the mausoleum was enlarged and converted into a church by the addition of an apse to the east. It was then turned into a mosque by the Turks. The interior was decorated with mosaics on a gold background of which a few have survived, particularly at the base of the dome.