The heartland of the Celtic Hallstatt culture was located around the Upper Danube; the first Celtic migrations were westward. Much of Gaul was occupied during the 6th century BCE, with rapid onward progress into the British Isles and Iberia. At these extremes, the regions of Celtic settlement became spatially isolated and culturally distinct; the Celtiberians of central Spain, for instance, did not adopt the artistic style of their eastern progenitors. In this initial expansionist phase, the Celts avoided the urbanized civilizations of the Mediterranean. But in the 4th century BCE, a new wave of migration would see them contest, and for a time prevail against, the Etruscans and Romans, settling in Lombardy and along the Adriatic. Their most spectacular thrust saw them conquer the Autariatae of Illyria; after defeating and decapitating the Macedonian king, Ptolemy Keraunus, they swept south to sack Delphi. A buccaneering offshoot of that venture went on to colonize Thrace and Anatolian Galatia.
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