Oppida are primarily associated with the Celtic late La Tène culture, emerging during the 2nd and 1st centuries BC, spread across Europe, stretching from Britain and Iberia in the west to the edge of the Hungarian Plain in the east.

[7][8][9] In regions north of the rivers Danube and Rhine, such as most of Germania, where the populations remained independent from Rome, oppida continued to be used into the 1st century AD.

In modern archaeological usage oppidum is a conventional term for large fortified settlements associated with the Celtic La Tène culture.

[12] In his Commentarii de Bello Gallico, Julius Caesar described the larger Celtic Iron Age settlements he encountered in Gaul during the Gallic Wars in 58 to 52 BC as oppida.

They were also political centres, the seat of authorities who made decisions that affected large numbers of people, such as the appointment of Vercingetorix as head of the Gallic revolt in 52 BC.

By 2011, only 21 of these had been positively identified by historians and archaeologists: either there was a traceable similarity between the Latin and the modern name of the locality (e.g. Civitas Aurelianorum-Orléans), or excavations had provided the necessary evidence (e.g. Alesia).

However, research has shown many of the localisations of Ptolemy to be erroneous, making the identification of any modern location with the names he listed highly uncertain and speculative.

[15][16] In particular, Dehn suggested defining an oppidum by four criteria: In current usage, most definitions of oppida emphasise the presence of fortifications, so they are different from undefended farms or settlements, and urban characteristics, marking them as separate from hill forts.

[21] What was swept away in Northern Europe by the Roman Conquest was itself a dynamic indigenous culture extending across the transalpine landmass, usually known today as that of the Celts.

The proto-urban Oppida – a Latin word used by Julius Caesar himself – remain one of the most striking manifestations of this pre-Roman northern European civilization.

[24] Central Spain has sites similar to oppida, but while they share features such as size and defensive ramparts the interior was arranged differently.

[14]: 25  According to Jane McIntosh, the "impressive ramparts with elaborate gateways ... were probably as much for show and for controlling the movement of people and goods as for defense".

Dry stone walls supported by a bank of earth, called Kelheim ramparts, were characteristic of oppida in central Europe.

[14]: 25  In western Europe, especially Gaul, the murus gallicus (a timber frame nailed together, with a stone facade and earth/stone fill), was the dominant form of rampart construction.

[26] For instance at the oppidum of Ulaca in Spain the height of the ramparts is not uniform: those overlooking the valley are considerably higher than those facing towards the mountains in the area.

The development of oppida was a milestone in the urbanisation of the continent as they were among the first large settlements north of the Alps that could genuinely be described as towns or cities (earlier sites include the 'Princely Seats' of the Hallstatt period).

Even in the lands north of the River Danube that remained unconquered by the Romans, oppida were abandoned by the late 1st century AD.