The Octagon was built west of the baths, and its main entrance faced the sea. Excavation of this monumental edifice began in 1950 and continued in stages until 1981, bringing to light the building we see today; further south, part of a peristyle not visible due to the construction of modern buildings was also excavated.
The building consisted of an octagonal hall and a monumental vestibule with two semi-circular niches on its narrow ends. The vestibule communicated via an opening in its south wall with a large peristyle court (width 47m., length 88m.). The front of this opening was formed with a triple arch (tribelon) and two columns, the latter preserved beneath the foundations of the building at 3 Isavron Street.
The porticoes, which were of different widths, had mosaic floors. At the north end of the east portico, which led to the marble staircase of the south corridor, there was a horseshoe-shaped niche framed by pilasters supporting a marble arch found during excavation of the site. This arch, known by the conventional name “The Small Arch of Galerius”, is on display in the Archaeological Museum of Thessaloniki.
The octagonal hall has an area of 875 square meters; on its interior it was configured with seven semi-circular niches opened at the base of the walls. The north niche (opposite the entrance) is larger than the others, and its masonry differs from theirs due to the way it was built and the brickwork ornament in its upper section.
The hall was covered by a dome 23 meters in diameter, at whose base concluded two winding staircases preserved today on either side of the entrance to the vestibule. Its height from floor to vault keystone was 29 meters.
The Octagon underwent many repairs and alterations both during its construction and over the course of its long period of operation. The building was probably designed as a regular octagon with a rectangular (on its exterior) vestibule. According to prevailing historical research, it was intended as the palace’s audience hall or throne room; later, it functioned as a Christian church.
The first building phase (to the death of Galerius in 311 A.D.) involved completion of part of the superstructure of the octagonal hall (to a height of 1.20 m.), the foundation of the vestibule, and the south peristyle, all of which belonged to the original design.
Work on the Octagon was suspended upon Galerius’ death; construction resumed after the Edict of Milan in 313 A.D. during the rule of Constantine the Great.
The construction of the building and widening of the north niche deemed requisite for functional reasons must have been completed during the time the emperor Constantine the Great stayed in the city. Completion of the interior decoration probably dates to the late 4th century A.D.
In a later building phase, a rectangular vaulted tomb with wall paintings dated to the second half of the 5th century A.D. was built beneath the floor of the north niche and within the ring of the foundations. Two small domed annexes were built flanking the north side of the octagonal hall. The east annex was later linked via a small portico with the palace’s central building complex, while a domed structure (still preserved) was built on the foundations of the west annex following the latter’s destruction. Construction of these small buildings on either side of the niche served the church’s operational requirements, and they may have been pastophoria.
The destruction of the Octagon is dated to the 7th century A.D., the era in which Thessaloniki was stricken by powerful earthquakes which destroyed most of its buildings. After the Octagon’s destruction, its vestibule was converted to a cistern that operated until the 14th century.