The construction of Thessaloniki’s first wall was contemporary with the city’s founding by Cassander (316/5 B.C.). The Hellenistic walls were abandoned after Macedonia became a Roman province (148/7 B.C.). According to sources and the archaeological finds, the first organized overhaul of the walls occurred in 253/4 A.D., when Thessaloniki was besieged by the Goths, and was completed before the construction of the Galerian complex. The Roman enclosure consisted of square towers which projected vis-à-vis the curtain wall every 45-50 meters (fig. 90). Reinforcements of the walls in places were made during the 3rd and particularly 4th centuries A.D. in order to confront the dangers of barbarian attacks more effectively. The most important reinforcement (still preserved today) was done in the lowlands of the eastern and western enclosure walls, where a new wall featuring towers shaped like isosceles triangles was built in contact with the outer side of the old one (building remains on Filikis Etaireias/ Tsimiski Streets, and on Egnatia St./ Syntrivani Square) (plan 1-58,72, plan 2-19, fig.91). Some scholars believe that this important fortification work belongs to the age of Theodosius I (379-395), while others date it to the mid-5th or early 6th century A.D.
The walls of Thessaloniki formed a quadrangle with two vertical sides extending down toward the sea (the eastern and western walls) and two parallel sides (the sea wall, the Acropolis wall). Archaeological investigation has determined that in actuality, two sea walls were successively constructed between the White Tower and I. Dragoumi Street. The inner one is identified with the route of Mitropoleos Street and belongs to the Roman period (3rd c. A.D.), while the other, which follows the axis of Proxenou Koromila Street, belonged to the Byzantine fortifications (7th c. A.D.). To strengthen the defensive system, there was yet another wall outside the western and eastern enclosure wall in the lowland sections of the city, the “outwork” (proteichisma) as the Byzantines called it (Filikis Etaireias Street).
The location of the eastern enclosure wall was probably determined by the stream that flowed down from the foothills of Mount Hortiatis, which emptied into the sea east of the White Tower.
An important role in the defensive system was played by the gates in the walls, which were directly connected with the city’s history. Known gates in the eastern lowland wall included the “Gate of Rome” (near the White Tower), the “Kassandreotiki (Cassandrian) Gate” or “Gate of Kalamaria” (at modern-day Syntrivani Square), the “Asomati Gate” (north of the Rotunda) and others.