It is a circular building with a dome was built in the III century as the mausoleum of Emperor Galerius. But the Emperor was buried in a different place, and the Rotunda was turned into a Christian Church. Thus, it is one of the oldest Church buildings in the world! At the end of the sixteenth century, the rotunda became a mosque, then a minaret was added.

Since 1912, the rotunda is a monument-Museum. The monument had marble revetment of the walls, which was not kept, and decorated with marvelous mosaics. It is unknown when exactly they were created. His mosaics are among the oldest surviving wall mosaics and probably belong to the time when the monument was converted into a Christian Church, i.e. the fourth century.