The Rotunda, part of Thessaloniki’s UNESCO World Heritage listing, was originally built in 306 AD as a Roman temple by Emperor Galerius, and may have been intended as his mausoleum. It was converted into a Christian church by Emperor Constantine in 326 AD, then a mosque (Hortacı Süleyman Efendi Camii) in 1590, and back to a church (Church of St. George) in 1912. It’s now an archaeological site but the Greek Orthodox Church maintains access for church purposes.
During my three visits, the inside of the Rotunda had lots of scaffolding all the way up to the dome where the mosaics were being restored. A simple altar sat on the far end.
The minaret and ablutions fountain are still intact. There are some interesting Jewish, Armenian, and Ottoman tombstones on the grounds. The Rotunda sits in close proximity to the Arch of Galerius and the Palace of Galerius.