The Thessaloniki webcam, refreshed every 10-minutes, offers a live view of the Arch and other areas around town. Also as a disclaimer, the descriptions below borrow heavily from content found on wikipedia, enjoy!
The Arch of Galerius and neighboring Rotunda (northeast) are early 4th-century monuments in the city of Thessaloniki. These two structures are elements of an imperial precinct linked to the emperor’s palace, the remains of which can be found to the southwest. These three monumental structures were connected by a road that ran through the arch, which rose above the major east-west road of the city.
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 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. Today, only a part of the monument still stands and there is little information on the period or the circumstances under which the rest of the arch was destroyed.
The Rotunda of Galerius (~125m northeast) is also known 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 Emperor Galerius, who was thought to have intended it to be his mausoleum. 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.
Small note on cache maintenance: I regularly visit Greece but in the meantime a local geocacher has kindly offered to assist if any issues arise between my trips.