"Modern historians, relying on reports by Caesar and others, have characterized the religion of the Celts in Gaul as spontaneous rites, in contrast to the well-planned cultic practices of the Greeks and Romans. Archaeologists have now revealed that the Gauls did, in fact, build permanent ritual sites at places like Gournay-sur-Aronde and Ribemont-sur-Ancre in northern France. Dated to the end of the fourth or beginning of the third century B.C., these cult centers were the work of warlike tribes called the Belgae, thought to have arrived in northern Gaul from central Europe at the end of the great Celtic invasions (1) of the fourth century B.C." Source: Archaeology (2001).
(1) - "Among the ancient European peoples were the warlike Celts--muscular, light-haired wanderers who probably came from the distant steppes beyond the Caspian Sea. By 500 BC they were living in northeastern France, southwestern Germany, and Bohemia. The Celts, who were also called Gauls, continued to migrate in all directions." Source: History-World.org.