The ancient city was organized and evolved within its fortifications which acted not only as the main line of defense against hostile attacks, but also as the conventional boundary between the town and the surrounding agricultural area.
The imposing late 4th century AD fortifications of Thessaloniki incorporated the earlier defense wall dating from the middle of the 3rd century AD, which it used as a buttress. This latter wall had been built hastily, using earlier architectural material, to protect the city from the raids of the Goths. The new walls had the shape of a trapezium and were about 8 km long. In the lower, more vulnerable section of the city, they were strengthened by an outwork and had stout triangular bulwarks placed at frequent intervals. Towers were built on the slopes of the hill, most of them rectangular. Triangular bulwarks alternated with rectangular towers at the most sensitive within it points of the Acropolis. On the side facing the sea, the city was protected by the sea wall, which also incorporated architectural members taken from the late Roman buildings in the Agora, probably for use in repairs made after the earthquakes of 620-630. This wall was low, and its outer face stood in the sea, and during the siege of Thessaloniki by the Saracens in 904, the general Petronios therefore planned to construct a submarine barrier by throwing sarcophagi and funerary stelai into the sea in front of the sea wall. The plan was not brought to completion, however, and the Saracens entered the city from the sea. The sea wall was later reinforced, before the siege of Thessaloniki by the Normans. 
At the south-west end of the sea wall was the large artificial harbor that was constructed by Constantine during his stay in Thessaloniki (322-323). The harbor was protected by a breakwater, referred to as molos in the 7th century and later as the tzeremboulo. There was a second, natural harbor called Kellarion on the east coast of the Thermaic Gulf. 
The Acropolis was appended to the exterior of the north-east section of the enceinte, and followed the general arrangement of the fortifications in the lower city, with alternating rectangular towers and defensive triangular bulwarks at its most vulnerable points. Evidence that the Acropolis was a later addition is furnished by the orientation of the towers on the dividing wall: these faced the interior of the Acropolis on what was originally the outer face of the city fortification wall. Inside the Acropolis there is a large cistern in the south-east section which was used for the city's water supply. On the north edge of the perimeter wall is the Fortress of the Heptapyrgion, to the south of which an Early Christian basilica has been excavated. 
The city has four main entrances: two Gates in each of the east and west walls, set at the ends of the two parallel main streets. The entrance to Thessaloniki from the west was via the Golden Gate, from which ran the main street of the city, the Via Regia of the Roman period, called Leophoros by the Byzantines, which lies beneath the modern Egnatia Street. The second gate at the west was the Letaia Gate, standing on the western extension of the modern Ayiou Dimitriou Street, beneath which ran a second ancient artery. In the east wall we know of the Cassandreia Gate (later the Kalamaria Gate) to which the Leophoros led, the New Golden Gate at the east end of Ayiou Dimitriou Street, and two other gates, the Asomatoi Gate and the Rome Gate. Smaller gates in the north wall of the city mainly served military needs. The Acropolis communicated with the city through a large gateway in the dividing wall, according to the Byzantine authors. Finally, there were two smaller gates in the Acropolis enceinte, communicating with the countryside. 
Large towers and forts acted as nodal points in the perimeter of the walls. Their present form is the outcome of major works undertaken by the Turks after the capture of the city to strengthen its fortifications and adapt them to the new requirements of artillery warfare. At the east end of the sea wall, where it joins the east wall, stands the White Tower, probably on the site of the 'eastern tower by the sea' mentioned by Byzantine authors.
Sultan Suleiman the Magnificent (1520-1566) built the Vardari Fort at the south-west edge of the enceinte, to protect the west side of the harbor. The east section of the perimeter wall of this fortress incorporated part of the west wall of the Early Christian period. At the north end of the east wall is the Chain Tower. In the 15th century this tower replaced the Byzantine Trigonion Tower, which was incorporated in its structure; before the eighteenth century, it was used as a gunpowder magazine and arsenal. 
At the end of the 19th century, the sea wall began gradually to be demolished, followed by large sections of the land walls in the lower, flat area of the city. The surviving fortifications, however, a length of almost 4 km, including the Acropolis and the Heptapyrgion, form an imposing complex. The line of the sea wall is being uncovered by excavations.