An oppidum was a large fortified Iron Age settlement or town create by Celtic. The Celtic settlements continued to be used until the Romans conquered Southern and Western Europe. 

In 194 BC a group of three hundred Roman citizens, with their respective families, settled on the slopes of an impervious hill in the northern part of the Sele plain (Ager Picentinus), giving rise to the city of Salernum. 

According to the historian Tito Livio, the foundation of this new colony takes place in a pre-existing fortified center , with the aim of garrisoning the territory previously inhabited by the Picentini. A Roman city worthy of the name had to have an amphitheater, forum, temples, theater, baths, aqueducts and, more generally, a building aspect of a certain type, with straight streets that crossed at right angles, possibly well paved and equipped with a sewer system, lined with brick houses.

Subsequently other settlers were sent to Salerno and in 199 BC Scipio transformed the Salerno maritime portorium into state customs. The aim of keeping the Picentini rebels in check justified the new direct Roman dominion in the gulf, which already belonged to Paestum, without arousing mistrust on the part of the nearby colonies and allied cities of Latin law. Among these we must include not only the very faithful Latin colony of Paestum, but also the ancestor of Roman Salerno, that is the ancient Etruscan city (Irnthi), to which the military aid provided in Rome before the battle of Canne must be referred. All this attention for Salerno on the part of Rome presupposes an economic and mercantile prosperity which is, in fact, attested by the pre-Roman archaeological materials discovered in the necropolis and in the ancient town, at the mouth of the Irno valley, in the Fratte district (Irnthi).

Once Paestum also decayed and the commercial function of the roman Porto Alburnus at the mouth of the Sele near the Heraion ceased, the sinus Paestanus will finally become the gulf of Salerno.

During the war between Rome and the Italic allies (Socii), the so-called “Social War” (91-88 BC), the rebel army, led by Papius Mutilus, conquered Salernum and enlisted slaves and prisoners.

"Salernum vetusta urbs, sita olim paulum supra mare" used to say Strabone (60 BC – 24 AD), so the ancient Salerno was built on the heights of the hill that dominates it, and not just close to the sea. Compared to the castrum, the city is located towards the sea, up to the south of the current via dei Mercanti, and expands towards the west, up to the course of the Fusandola stream, and towards the east, up to the outlet of the same via dei Mercanti on today's wide Sedile di Portanova. Perhaps the forum has been set up off Largo Abate Conforti, certainly in the area today of San Pietro a Corte the baths are built and the necropolis is located beyond the eastern walls.

The earthquake in 79 AD damaged also Salernum. The restoration works to public buildings, thanks to emperor Titus’ intervention, are linked to this circumstance and there is also a reference to them in an inscription found in 1948 in the necropolis of Vittorio Emanuele Avenue.

Under the Emperor Diocletian, in the late 3rd century AD, Salernum became the administrative centre of the Bruttia and Lucania province.

An important period in the life of Salernum seems to be the age of Constantine (306-337 AD). Two statue bases have been found in Largo Abate Conforti, perhaps corresponding to the tribunal area, one related to a statue dedicated by the city senate to emperor Constantine, the other put in honor of his mother Elena by Alpinio The Great, governor (corrector) of the province of Bruttia and Lucania

Between the end of the 4th century and the early 5th century AD, a calamity shocked Salernum. A strong flood caused by a stream that passed through the urban area made the situation so difficult that many people chose to leave the city. Following these floods, it has always been preferred not to remove the layers originated by the disastrous events but to carry out a series of fillings; it resulted in an often insurmountable obstacle for archaeological investigation.

WHY DID ROME DEVELOP ON THE HILLS AND NOT ON THE PLAINS?

One of the most surprising things about ancient Salerno is that it developed first on the hills, not on flat land in the valleys.

Why would anyone want to build a city on the hills? Imagine carrying a jug of water on your head up several hundred steps every day!

You definitely wouldn't need to go to the gym three times a week – going to get some water would be a great workout.

So, why did Salerno develop on the hills and not on the plains?

There were several factors that came into play in determining where people lived in ancient times. These factors were focused on providing the basics of life, such as food and water.

But above all, defend your city in times of war.

War is the key factor, because this is the main reason why men in ancient times built on hills. A high hill with steep slopes and a defensible water source near a valley where crops could be grown made it difficult for enemy armies to conquer the city.

In ancient times, the issue of keeping the family alive in times of war was of critical importance. It was the key factor why cities were built on hills.

The best place to defend a city was on top of a high hill with steep slopes. But sieges were also a problem, so it was necessary to have a continuous source of water and the possibility of storing food within the city walls.

The ancient Romans built Salerno on the hills, not in the valley, because they could defend themselves better on the hills. On top of the hills they added tall, large stone walls to make it even more difficult for their enemies to enter the city to kill or capture them or their families.

If your city has been besieged by a large army for a long time, you need a source of fresh water.

They also built on hills near valleys where they could grow grain and other crops so they would have food for daily living, as well as food to store in times of war.

The city of Rome itself is therefore built on a series of hills, including the Palatine, the Capitoline Hill and the Aventine.

While the cardo and decumanus formed the primary street layout in many Roman cities, the presence of stairs and ramps was common in areas with elevation differences. These stairs and ramps were integrated into the city's infrastructure to ensure accessibility to various parts of the city. Many Roman towns or settlements built on hills not strictly follow the traditional cardo and decumanus grid plan but are known for their use of stairs, ramps, or other features to navigate the elevation changes:

Terracina (Italy): Terracina, located on the Italian coast, is known for its ancient Roman town built on a hill. It includes steep streets and staircases connecting different levels of the town.

Sutri (Italy): Sutri, in Italy's Lazio region, is an ancient town with a hilly landscape. It has a mix of staircases and winding streets.

Sperlonga (Italy): Sperlonga, located on the Tyrrhenian coast, features a picturesque Roman-era town with winding streets, staircases, and terraced houses.

Segni (Italy): Segni, located in the Lepini Mountains, is known for its ancient Roman layout, which includes stairs and steep streets.

Nemi (Italy): Nemi, overlooking Lake Nemi, is famous for its ancient Roman temple to Diana and includes stepped streets and pathways.

Anacapri (Italy): Anacapri, on the island of Capri, has a historic town with narrow streets and steps connecting different levels.

Terrasson-Lavilledieu (France): In France, Terrasson-Lavilledieu is known for its hilly layout, ancient Roman roads, and staircases.

Gubbio (Italy): Gubbio, in Umbria, Italy, is an ancient town with a steep and picturesque layout featuring stairs and winding streets.

These towns showcase the Roman ability to adapt to different geographical settings and create functional settlements on hilly terrain, often with the use of stairs and ramps to facilitate movement within the town.

Stairs, ramps, and other structures were used to connect different levels of a city built on hilly or uneven terrain.