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.

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