The Emperor Galerius chose Thessaloniki as his capital in 297 and began construction on a massive palace complex. The modern city is built all around the remains. In this top picture, we are down in the ruins (you can tell how old things are by how far below the normal street level they are). In the background, you can see the "Rotunda," which was originally built as a temple to the patron gods of Diocletian's imperial tetrarchy (of which Galerius was one). In the 5th century, it became a church filled with icons of saints martyred by Diocletian and Galerius, ironically. Today it is still used on occasional feast days. To the right of the Rotunda (St. George's) is the University.
This is a shot of the ruins of the palace from the current street level. The palace seems to be a hang-out for feral cats.