The walls of Thessalonica have a long history. Though the White Tower is perhaps the best known of all defences, the fortifications date back to the city’s foundation in Hellenistic times.   The wall was shorter in circumference at the time, since the settlement only took up a small area, possibly extending north from Egnatia Street up to Olympia Street. All that has been excavated of the Hellenistic wall is a small part of the eastern section, built of the well-known local green slate.

The main fortifications around the city date to late antiquity. The first construction phase was completed in the mid-3^rd century to fend off Gothic raids. Subsequent phases have been attributed to Constantine the Great , Julian and Theodosius I, emperors in the 4^th century. All three reinforced both sides of the older wall, the towers and the spaces in between by adding successive masonry skins as overlays. In the phase under Theodosius I, the east and west parts of the wall had alternating triangular and rectangular towers spaced at short intervals to strengthen defence; the south section of the west wall was built of figurines from the city’s theatre / stadium and other marble spolia . An inscription records that in the time of Emperor Maurice, repairs to the sea wall were sponsored by Eusebius, Bishop of Thessalonica. In the reign of Heraclius the northern part of the wall was reinforced with mixed masonry (opus mixtum), consisting of alternating layers of bricks and rubble. This was punctuated by numerous brick crosses, possibly to offer extra protection. The citadel was probably built once the city had recovered from the Saracen occupation. It was crowned by the Eptapyrgio (“Seven-tower”), now a complex of buildings dating from mid-Byzantine to Ottoman times.

The city walls were repaired many times; inscriptions proclaimed the names of those overseeing the works or the date they were completed, as for instance: «† Επί Μαρίνου βασιλικού πρωτοσπαθαρίου εκαινουργήθη ούτος ο πύργος, επιστατούντος Κακίκη βασιλικού στράτορος, έτει από κτίσεως κόσμου ‚ϛτο’ ινδικτιώνος ι’» [“In the time of royal protospatharios Marinos, this tower was renovated under the supervision of royal strator  Kakikis, Anno Mundi 6370, in the 10^th Indiction”]. The Year After Creation or Anno Mundi (5508) was the official calendar the of Byzantines, while the indiction was a year of a period of fifteen years, defined for tax purposes (the date is thus 6370-5508 = 862). Other inscriptions mention the foremen or sponsors of repairs to the wall in the 12th century, in 1316, in 1335-1336, and in the time of Anna Paleologina and of Manuel II Palaeologus .

Renowned installations include the Chain Tower on the northeastern rim of the ramparts, and the Vardar Fortress on the southeastern rim. Located at the intersection of the northern and eastern walls, the Chain Tower replaced an earlier Byzantine structure known as the Triangular Tower. It is mentioned in sources as Zincirli Kule (Chain Tower) or Kuşaklı Kule (Girded Tower), on account of the stone cornice that surrounds the core of the tower like a chain. Τhe Vardar Fortress is located to the west of the harbour, and is also known as Top-hane or Tabak-hane, after the tanneries once in the area. It was built by Sultan Suleiman the Magnificent , during a short stay in the city in the winter of 1546. The fortress consists of a horse-shoe shaped enclosure adjoining the west part of the city walls, on the south side of which lies the rectangular Relief Tower, named after a Hellenistic relief immured in the eastern wall.

Possibly the best known of all fortification towers in Thessalonica is the White Tower, built where the east walls meet the sea walls. It is believed to date to the early years of Ottoman rule. Dominating the city’s shore, it is a cylindrical stone structure standing 34 metres tall, with a ground floor and six upper floors, each of which has a large, round central hall opening onto smaller quadrangular rooms built into the thickness of the outer wall. The floors are connected by a spiral staircase leading up inside the  same walls. In the initial construction phase the tower was surrounded by a polygonal wall with outposts at the corners. During the Ottoman period it served as a prison for long-term convicts, and was known as the Blood Tower or Torture Tower.