The construction of the Salonitan city walls took several centuries. The earliest part of the city was surrounded by walls as early as the second century BC. During the Pax Romana the city expanded to both east and west.
The town’s elliptic core was surrounded by walls built over several centuries. They are quite well preserved on the northern side, where there are about a hundred square towers. The walls extend from the amphitheatre to the town northeastern gate, by which the road led into the hinterland, firstly to Klis, and then by one branch to Andetrium and by the other to Osinium (Sinj) and further on to the Cetina, Tilurium and deep into the Balkans. The oldest wall remains, probably date from the Roman Republic times, were built in large stone blocks (megalithic walls, often from the Hellenistic and early Roman times). These are situated in the centre of the town, at their eastern end there is preserved the somewhat later town gate known as Porta Caesarea. This was built in the first century A.D. It is hard to say when the first fortifications were built, they were probably intended to protect the trapezoidal town, where the Roman consul Cecilius Metel wintered in 119-118 while fighting here against the Illyrians. At the time of August’s peace, after conquering the Delmats and other Illyrian tribes, Salona started spreading out from its old, original town core, both westward and eastward. The eastern, monumental town gate thus remained right in the town centre, losing its original purpose, like the gate in the western walls of which no remains have been preserved. In the fourth century, above the eastern gate, a decorative keystone was built in, showing the goddess Tycha.
During the reign of Emperor Marcus Aurelius around 170 A.D., under constant threat of Germanic tribes the east and west suburbs were included in the walls which were fortified with at least 90 towers. Some parts of existing buildings were used in the extensions to the walls thus making them an integral part. Total circumference of the elliptical shape of the walls was approximately 4 km, with varying width from 1.9 to 2.5 metres.
During the reign of Emperor Theodosius II in the early fifth century all the towers were reconstructed, as witnessed by an inscription on the walls. Furthermore, in the first half of the sixth century, in order to improve city’s security and defence system, triangular shaped endings were added to some square-shaped towers. Such examples are visible today on the northern side of the Urbs orientalis. Best preserved part of the oldest part of the city (Urbs vetus) is eastern wall and Porta Caesarea with two octagonal towers and three passages; one for cart traffic and two for pedestrians on each side of the wider passage. Central passage was probably equipped with a movable grid, as indicated by grooves on side pylons. Porta Caesarea was constructed using large regular stones primarily for fortification purposes. After eastern and western expansion had occurred, the gate lost their primary purpose and became carrying construction of the aqueduct. According to Kähler reconstruction, the gate had two floors, of which the top one was very elaborately decorated with half columns, composite capitals, and window openings. Within the gate there was small courtyard for defense purposes.
The largest part of the earliest walls that protected the eastern part of the town were erected in about 170, this being certified by two inscriptions cut into a stone slab and built into the outer, northern, side of the walls, near the Porta Andetria. The two complete and valuable inscriptions say that a Delmatian cohort (cohors secunda Delmatarum), commanded by the tribune Granius Fortunatus, built 800 (Roman) feet and the 2nd and the 3rd sections of the legion, supervised by the centurion Publius Elius Aminitianus, 200 feet of walls and several towers in the north part of the town. One lost inscription read that, at the same time, the first Dalmatian cohort erected 800 feet of wall and one tower. That would be equal to about 430 metres of walls in total.
Within the ellipse, the majority of the Salonitan monuments are situated there, outside it there were pagan and Christian cemeteries, the latter with cemetery churches. Of course, on the large area of the Roman land plotting (colony centurion, both westward and eastward, to the present Trogir, Split and Stobreč, there were residential and industrial buildings, cemeteries, industrial installations, etc., confirmed by discoveries appearing at almost every present-day construction works.