What is "heidengraben"

Heidengraben ("pagans' moat") is the name given to the remains of a large Celtic fortified settlement ( oppidum) dating to the Iron Age, located on the plateau of the Swabian Jura (Schwäbische Alb) in the districts of Reutlingen and Esslingen in Baden-Württemberg, Germany. The settlement was in use from about the late 2nd century BC to the early 1st century BC. By surface area, Heidengraben is the largest oppidum in all of mainland Europe.