Salernum has grown on itself in a continuous overlapping of the levels of attendance. This succession, commonly referred to as "palinsesto", it has not spared any of the buildings of the historical center, under which lies the whole old Roman settlement. Anyway the first documented Roman presence in the area was not a city but a Castrum, a stable encampment fortified by legionaries. The position of this stable was probably in Plaium Montis.

Salernum as oppidum is, for the first time, remembered with certainty in the year 197 BC when (with the lex Atinia de coloniis quinque deducendis), Rome decided to found five civium colonies on the coasts of Campania and the neighboring Lucania Tyrrhenian ones, including a brand new close the Castrum Salerni.

So in the first years of life of the "Castrum quo in loco nunc oppidum est", the new Roman trade colony (made by simple citizens) and the ancient military castrum had to coexist, since the latter probably performed the task -the trunk of the Rhegium-Capuam had not yet been built- of mediate the city's relations with the outside world.

Depopulating the Irpinia lands, an intense migratory flow headed towards the Roman Salernum caused by the transformation of the ager confiscated by the Romans into large estates in the hands of local or Roman aristocracies with large companies - rustic villae - managed mainly by slaves and with the resumption of transhumantic pastoralism.

The port city grew in importance for its trade and in the 4th century AD it was already superior to Paestum as a center of political power for Lucania et Bruttii: the "Sinus Paestanus" became "Sinus Salernitanus".

The civil system, distinct between the Order and the People (Ordo Populusque) provided that the administration of the peculiar public and the care of public affairs remained with the citizens and the nobles, and that the offices remained divided between the nobility and the people. The certainly functional buildings of Roman propaganda are those intended for public events, and in particular the theaters and amphitheaters. Unfortunately, no monumental remains of the time have come down to us, because the Roman remains lie at a considerable depth compared to today's city, buried by catastrophic floods such as that of the 4th / 5th century AD.

Salerno at the time of the Empire already had a reputation as a holiday city. Quinto Orazio Flacco, in the 15th epistle published in 20 BC, having an eye disease, perhaps ophthalmia, decides to go on vacation to the sea and wants to escape from too well-known places such as Baia. He then turns to his friend Numonio Vala and asks him: "What is the climate in Salerno, Vala? What people live there? And how does the road look like?". What interests the Lucanian poet is a pleasant climate, the possibility of eating good fish, "a generous, lovable wine, which frees me from thoughts, runs in my veins and in my heart full of hope".

Here, in 43 BC, was also the villa of the consul Caio Plozio Planco, who, in order not to give up the ointments with the intense scent of the rose of Paestum, prohibited by the Roman Triumvirs, Licinius and Julius Caesar, preferred to be killed.

The Roman Salerno lasted for 660 years that is, it has had plenty of time to develop with more or less important public, private and religious buildings: being the seat of several temples (dedicated to Bacchus, Venus, Juno and Priapo), it received the title of "Collegio degli Augustali" demonstrating the great importance it held during the Roman period.