Rotonda Thessaloniki Greece

The monument was built by Galerius Ceasar in ca. 300 A.D. This is the oldest structure still in use in the city. Initially it was a place to worship the mysterious Samothracian Kabirii (sons of Hephaestus and the statuettes around the rotunda are of the Cabeiri) or Zeus. It has since attracted saints, dervishes, mystics, and lately devotees of modern art and jazz.
It was converted to a christian church ca. 390 (hence the apse) and to a mosque in 1591. It is well known for its mosaic decoration, one of the best samples of early christian art.