Parts of the Palace of Galerius can still be seen and visited for free in a square in modern Thessaloniki, though much if it is covered over by the surrounding blocks of flats.
Galerius was one of the four tetrarchs, (one of two junior emperors, the other being Constantius) appointed by Diocletian in 293 AD to rule a quarter of the Roman Empire alongside his co-emperor, Augustus. As well as building the Palace and part of the city walls, he also built a hippodrome, triumphal arches and the Rotunda.
On the ground it is difficult to get a sense of what the different parts of the ruins relate to the Palace as a whole, though as with many sites in Greece it is well signed and the information boards are written in excellent English.
Much of the lower part of the city of Thessaloniki was destroyed in the fire of 1917 and had to be re-built. It is interesting to note that the lower storey buildings (3rd from the right in the top right hand corner of the above photograph) are typical of the housing that was re-built after the fire. Much of this old style of building was however destroyed in the German bombardment of the city during the Second World War or replaced in the building boom from the 1950s onwards.