Celtic hilltop forts, often called oppida (sing. oppidum), after the Latin name given to larger settlements by the Romans, were built across Europe during the 2nd and 1st century BCE. Surrounded by a fortification wall and sometimes with outer ditches, they were usually located on high points in the landscape or on plains at naturally defensible points like river bends. One of the most famous examples of an oppidum is Alesia, which resisted a siege in 52 BCE by Julius Caesar (l. 100-44 BCE) before its chief Vercingetorix (82-46 BCE) surrendered. Other notable oppida include Gergovia in Gaul, Heidengraben in Germany, and Maiden Castle in Britain. Following the spread of the Roman Empire in the 1st century CE, many oppida were abandoned, but some, like Alesia, remained inhabited into the Middle Ages or retained a religious significance to the local population.
The Celts of Iron Age Europe, thanks to developments in agriculture and manufacturing, were able to prosper and so small urban settlements became more populous. Increasing trade with the Romans and the acquisition of slaves and precious goods resulted in an increase in local rivalries and intertribal warfare as tribes competed for resources. The Celts then built fortifications at easily-defended locations like low hills, mountains, and river bends. There may also have been some religious significance in the choice of a fortified site as attested by multiple votive artefacts found at many of them. They were not necessarily places of permanent occupation, although some were used as such. Rather, many were, in times of war, used as a point of refuge. People had lived in naturally protected spots since the Neolithic period or even earlier, but the Celtic hilltop forts of the 2nd and 1st century BCE were notable for their size and human-made defences. Many oppida spread over an area of several square kilometres (1000 acres). These forts are usually called by archaeologists the Latin name oppida (sing.: oppidum). However, to the Romans, oppidum simply meant ‘settlement’ and could be applied to any area of occupation larger than a mere village. The chosen area for an oppidum was typically encircled by walls with restricted access permitted via gates. The walls, at least at sites west of the Rhine and in southern Germany, were constructed using a technique the Romans called murus gallicus because they first came across it in Gaul. The method was to create a framework of wooden beams attached together using long iron nails. Within the framework, layers of rubble and earth were laid down with more intersecting beams for added strength. The whole was then faced on both sides with cut stone blocks through which the ends of the horizontal beams were still visible. In oppida in Bohemia and Moravia, another method was employed, the post slit wall (Pfostenschlitzmauer or Kelheim wall) where a wall of vertical wooden posts with interconnecting horizontal beams and dry-stone-work was backed by a ramp-like structure of rubble and earth. A third type of defensive wall was prevalent in northern Gaul and southeast Britain. This was the Fécamp type, named after the oppidum of that name.