Some say that Galerius built this massive brick-and-stone structure as his immodest mausoleum, while others think he intended it to be a temple, perhaps of Zeus. Later in the 4th century, the Byzantine emperor Theodosius the Great converted the Rotunda into Ayios Yioryos (Church of St. George), and began the ornamentation of its 6m-thick (20-ft.) walls with mosaics. When the Turks conquered Thessaloniki in the 15th century, they converted the church into a mosque (you'll see one minaret left from this period) and destroyed many of the mosaics.
The Rotunda reopened in 1999 after more than a decade spent repairing the damage done by the earthquake of 1978. Artisans and archaeologists worked with literally millions of tesserae, or tiles, to restore the mosaics. Now you can see (especially if you bring binoculars) indigo peacocks, garlands of flowers and fruit, and blond, curly-haired saints and martyrs (and martyred saints) in the mosaics that cover the walls. Try to visit on a Wednesday, when the neighborhood street market stretches for blocks around, and ask about special exhibits or concerts. Allow an hour to explore.