Itri lies along the route of the Appian Way, between Fondi (with which it borders to the west) and Formia (with which it borders to the east). A strategic position between the mountain ridges that slope down towards the Port of Gaeta and Torre Mola, which later became Formia.

Inhabited since prehistoric times, the first real settlement dates back to the Romans when they built the Appian Way, which, linking Rome to Capua and later to Brindisi, was the obligatory passage for all those travelling south.

Initially intended as a refreshment station for travellers and their horses, in the Middle Ages the town underwent urban development, especially when, in 882, work began on the construction of the defensive structure that was especially useful to Gaeta as a defence against the Lombards attacking from the hinterland. At the highest point, on the hill of Sant’Angelo, stands the Castle, and around it, as it slopes down, the mediaeval village develops.

The settlement therefore initially grew up around the castle in the upper part and then expanded towards the Via Appia in the lower part. The mediaeval village was protected by three walls that are still partially visible.

It became part of the Duchy of Gaeta until 1073 when it was annexed to the county of Fondi. In 1591 the county of Fondi, of which Itri was part, passed to the princes of Stigliano. From the 13th century until 1861 it was part of the Kingdom of Naples (later Kingdom of the Two Sicilies) within the ancient Province of Terra di Lavoro. The territory of Itri bordered the Papal State and, over time, its defensive importance lost its value. Eventually, the castle was abandoned and was easy prey to the brigandage that plagued its border areas.

In 1927, by decree, the legislator voluntarily abolished the province of Terra di Lavoro, the largest in the kingdom, assigning the territory of Itri to Lazio, in the province of Rome. It was in 1934, with the establishment of the province of Littoria – later Latina – that Itri was assigned.

During the fighting of the Second World War, Itri was at the centre of an area of fierce fighting known as the ‘Gustav Line’ and was heavily bombed, destroying 75% of the town. The castle also suffered serious damage to the roof, the polygonal tower, and the battlements.