The Galerian Complex, the most important monumental group in Thessaloniki, was built at the turning-point of two worlds, the Roman and Byzantine. Its erection began in the late 3rd century-early 4th century AD, when the Caesar Galerius (293-311 AD) chose Thessaloniki as the seat of the eastern part of the Roman Empire.

During Early Christian times, important 4th century emperors occasionally stayed in Thessaloniki due to its significance and geographic location, situated between Rome and the New Rome - Constantinople.

Significant building remains of the complex came to light in excavations carried out during the second half of the 20th century. Some of these, like the Apsidal Hall  and the buildings at the archaeological site in Navarinou Square, are visible and open to the public, though most have been buried due to the reconstruction of the historic city center.

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The Basilica was a magnificent building which functioned as a reception and audience hall. It had a rectangular plan with a semi-circular apse on its south side, and a vestibule with a mosaic floor on its north. The building measured 24 x 67 meters on its exterior, rose to a height of around 30 meters, and was roofed. Its longitudinal axis lay parallel to the west side of the Hippodrome. Between the two structures was a road nine meters in width. The Basilica faced the Apsidal Hall, with which it shared a common courtyard with surrounding portico. The Basilica had a single interior nave and its walls boasted marble revetments.

The construction of this building was completed in stages, and it was subjected to many alterations, chiefly in the floor structure and apse niches. Today, only the west wall masonry and the greater part of the apse of this monumental building are visible; the rest lies buried beneath the pedestrian walkway of Navarinou Square and D. Gounari Street.

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West of the Basilica is another important building unit -  a central building complex - that belonged to the palace. This area was inhabited before the palace complex was built, as was determined by the building remains of luxurious houses dating to the 2nd and 3rd centuries A.D. that came to light during recent excavations in the nineties. The peristyle consists of a building measuring 30 x 40 meters with 11 rooms organized around three sides of a nearly-square courtyard, surrounded by a porticus. There was a fountain in the center of the courtyard. In the form it is preserved today, this building formed a closed whole surrounded by wide covered corridors with mosaic floors.

There were gates in the masonry of the corridors allowing communication with the Basilica and other palace units to both north and south. In the middle of the south passageway was a monumental marble staircase that led to a corridor nine meters wide with mosaic floor (fragments of which are preserved); this in turn probably led to the palace’s main entrance towards the sea. On either side of this corridor (which was not excavated to its entire length) were the palace baths and the Octagon.

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The total area covered by the baths of the complex is not known, because their excavation has not yet been completed. The building was subject to many alterations in antiquity involving changes in the use of individual spaces. The ground plan of the rooms, as this is preserved today, belongs to the final building phase.

The entrance to the baths was along the main axis linking the south entrance of the palace with the central building complex. A square vestibule with a mosaic floor led to a large rectangular reception room adorned with colored marbles on its walls and floor. There were shallow niches in the north wall, while at the east end there was a cold-water pool (frigidarium) that was later done away with, and a semi-circular decorative fountain built in its center.

Two doors in the room’s south wall led to two further areas: an octagonal one with a hexagonal warm-water bath (tepidarium), and a rectangular space with an apse on its west side, in the center of which there probably would have been a round marble basin for producing steam. In a later building phase, the apse was transformed into a bathtub. The rooms with hot water baths must have occupied the southern part of the building, which continues beneath the buildings in Navarinou Square. These rooms were heated by the furnace in an adjoining ancillary space which later changed use. The toilet (latrina) was near the entrance to the baths, south of the vestibule.

North of the room is preserved a vaulted cistern which was built later, and to its east is another building which was probably the bath’s original cistern.

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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 AD) 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 AD 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 AD.

In a later building phase, a rectangular vaulted tomb with wall paintings dated to the second half of the 5th century AD 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 AD, 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.

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The “Apsidal Hall” was probably the last building on the north belonging to the palace. Assuming this, the boundary of the complex is identifiable with the wall preserved today at the northern edge of the archaeological site.

The hall was built on a notional north-south axis, with another magnificent building, the Basilica, at its southern end. Remains of this building are visible in the eastern section of the archaeological site in Navarinou Square. The “Apsidal Hall” had a southern orientation and its exterior initially had the form of a basilica, i.e. a rectangular hall with an apse on its northern side.

Its interior was divided into two rooms which opened onto one another: a rectangular vestibule and a large hall with two niches on its south side. On the north, it ended in a raised apse.

The hall was heated by means of hypocaust channels still preserved below its flooring. In the middle of the masonry of the apse, there was a vaulted opening (a furnace, praefurnium) where wood was burned to produce warm air. Parts of the two vertical clay pipes used to draw off smoke are preserved encased in the apse’s masonry.

Both of its rooms carried rich decoration which is fragmentarily preserved. Their walls had white and colored marble revetments, and their floors were covered in marble laid in the opus sectile technique (marble tiles of different colors fitted together to form geometric patterns).

In a later building phase, probably in the 6th century, another hall (A) was added south of the vestibule. Given that a large part of it is buried beneath the paving of A. Svolou Street, its length is not known.

This hall, which probably led to a peristyle courtyard had a mosaic floor and luxurious interior decoration. Its north wall bore mural painting to the height of the doorway. Fragments of the mural painting found in the excavation of the monument featured representations of delicate wavy tendrils and red florets. Above the wall painting, the wall was covered with a mosaic that carried a Latin inscription, fragments of which are preserved.

In 1939, east of the new hall (A) at the site of the modern building located at 25 Dimitriou Gounari Street, the remains of a room (10.4 x 10m.) with marble decoration were found. According to E. Dyggve, this room, the existence of which is confirmed by an excavation drawing preserved in the Ephorate’s Archives, was part of the Hippodrome’s imperial viewing box (kathisma).

Modern scholarly opinion maintains that the “Apsidal Hall” was a triclinium, that is, it was used for banquets and other ceremonies connected with the presence of the emperor and his retinue in the Hippodrome.