The city of Thessaloniki, since its erection in the Hellenistic period until the second half of the 19th century and the last years of Ottoman occupation, was organized and developed within its walls, which both protected it from prospective conquerors and formed the boundary between the urban area and the rural area. King Cassander of Macedonia, Thessaloniki’s founder, saw to fortify the city with new defensive perimeter, which survived up to the Roman era. Ruined parts were repaired in the mid-1st century BC under the pressure from the expected enemy attack while in the mid-3rd century AD in order to face the imminent danger of invading Goths, there was constructed a single wall with square towers axially placed. After the end of the 3rd century, the walls of Thessaloniki were reinforced at times where they were most vulnerable, either because of their small thickness or because of their low height. In the late 4th century and during the reign of Emperor Theodosius I, there were made extensive and imposing fortification works. In later years, the defensive perimeter was strengthened even further, resulting in the middle of the 7th century to be extremely compact and have towers in very frequent spaces, which made Thessaloniki truly impregnable by land. It is obvious that in the following centuries the earthquakes and invasions have created the need of many and successive repairs and interventions in the fortification. After the city's capture by the Turks in 1430, there were erected at key points in the perimeter of the walls great castles and fortified forts, such as the White Tower and the Fortress of Eptapyrgiou.
The walls of Thessaloniki surrounded the city on all four sides (east, west, north and south), having a trapezoid shape and a total length of about 8 km. Their height, on average,ranged between 10 and 12 m and their thickness reached 5 m. In lowland areas of the city, who were most vulnerable, the defensive perimeter had outwork and strong triangular overhangs. On the slopes of the hill, there were constructed mainly rectangular towers. On the southwestern side of the walls there was a large artificial harbor, created in the 4th century by Constantine the Great, while on their northeast side there was attached the Acropolis, which had a separate fortification consisting of triangular overhangs, that alternated with rectangular towers. The main gateways in Thessaloniki were four: two were opening on the west wall, the Golden Gate and the Litaia Gate and two were formed on the east wall, the Kassandreotiki Gate (or Gate of Kalamarias) and the New Golden Gate. Smaller gates existed in other parts of the fortification, which served mainly military needs. In 1873, in relation to city the embellishment from the Turks,it began the systematic demolition of large sections of the walls, with the south wall - called the marine (as it was growing in the side of the sea) - to be completely destroyed. However, today the walls of Thessaloniki survive in length of about 4 km and compose an impressive building set.