Celts are a mix of different peoples and countries whom settled in a sizable section of European countries between the Iberian Peninsula and Britannia. They'd an homogeneous religion, but always were politically split. The better known ethnic group is the Gauls. Fierce and hard-fighting, in 386 B.C. the Senones Gauls led by Brennus put Rome under siege and occupied it. These were later subjugated by the Romans.