Arch Of Galerius :
The Arch of Galerius, stands on what is now Egnatia & Dimitrios Gounari Street. The arch was built in 298 to 299 AD and dedicated in 303 AD to celebrate the victory of the tetrarch Galerius over the Sassanid Persians and capture of their capital Ctesiphon in 298. The structure was an octopylon (eight-pillared gateway) forming a triple arch that was built of a rubble masonry core faced first with brick and then with marble panels with sculptural relief.
The central arched opening was 9.7 m wide and 12.5 m high, and the secondary openings on other side were 4.8 m wide and 6.5 m high. The central arch spanned the portion of the Via Egnatia (primary Roman road from Dyrrhacium to Byzantium) that passed through the city as a Decumanus (east-west major street). A road connecting the Rotunda (125m northeast) with the Palace complex (235m southwest) passed through the arch along its long axis.
Only the northwestern three of the eight pillars and parts of the masonry cores of the arches above survive: i.e., the entire eastern side (4 pillars) and the southernmost one of the western pillars are lost. Extensive consolidation with modern brick has been performed on the exposed masonry cores to protect the monument. The two pillars flanking the central arched passageway retain their sculpted marble slabs, which depict the wars of Galerius against the Persians in broadly panegyric terms.
Rotunda of Galerius :
The Rotunda of Galerius is 125m northeast of the Arch of Galerius. It is also known (by its consecration and use) as the Greek Orthodox Church of Agios Georgios, and is informally called the Church of the Rotunda (or simply The Rotunda). The cylindrical structure was built in 306 AD on the orders of the tetrarch Galerius, who was thought to have intended it to be his mausoleum. It was more likely intended as a temple; it is not known to what god it would have been dedicated.
The Rotunda has a diameter of 24.5 m. Its walls are more than 6 m thick, which is why it has withstood Thessaloniki's earthquakes. The walls are interrupted by eight rectangular bays, with the south bay forming the entrance. A flat brick dome, 30 m high at the peak, crowns the cylindrical structure. In its original design, the dome of the Rotunda had an oculus, as does the Pantheon in Rome.