Piacenza lies on the right bank of the river Po, at a crucial crossroads in the south-west area of the Po Valley. The first settlements date back to the stone and bronze ages. Gauls and Etruscans are likely to have settled in the area at a later stage, but there are no certain traces left.
The earliest urban settlement may be traced back to the year 218 B.C., when six thousand Romans founded the colony of "Placentia". They left their mark in the layout of the town, which has a square plan organized around two main intersecting streets, called cardo (north to south) and decumanum (east to west), and a web of side streets at right angles to each other. The strength of the colony was first put to the test in the second Punic War, when the Romans fought a fierce battle against Hannibal on the river Trebbia.
In republican and imperial times, Piacenza became an important centre with a flourishing river port, and from 187 B.C. it was the end town of the Via Aemilia, a main road at the foot of the Appennines, which joined the Rome-bound Via Flaminia at Rimini.
With the beginning of the Christian era, the community consecrated modest chapels to its first martyrs, which later grew into important churches. During the Middle Ages the town was ravaged quite a few times, fell under the power of the Germanic invaders and suffered the effects of the war between the Goths and the troops of the Eastern Roman Empire. After being under the rule of Ostrogoths and Byzantines, Piacenza became the administrative centre of a Longobard dukedom, but its true recovery started in the ninth century, under the Franks.