Strabo cites Ephorus's belief that there were Celts in the Iberian peninsula as far as Cadiz. The material culture of the north-western regions of the Iberian Peninsula showed continuity from the end of the Bronze Age (c. 9th century BCE) until it was subsumed by Roman culture (c. 1st century BCE). It is associated with the Celtic tribal groups the Gallaecians and the Astures. The population predominantly practiced transhumant cattle-herding, protected by a warrior elite, similar to those in other areas of Atlantic Europe, centered in the hill-forts, locally termed castros, that controlled small grazing territories.Settlements of circular huts survived until Roman times across the north of Iberia, from Northern Portugal, Asturias and Galicia through Cantabria and northern Leon to the Ebro River.

Celtic presence in Iberia likely dates to as early as the 6th century BCE, when the castros evinced a new permanence with stone walls and protective ditches. Archaeologists Martín Almagro Gorbea and Alvarado Lorrio recognize the distinguishing iron tools and extended family social structure of developed Celtiberian culture as evolving from the archaic castro culture which they consider "proto-Celtic".

Archaeological finds identify the culture as continuous with the culture reported by Classical writers from the late 3rd century onwards (Almagro-Gorbea and Lorrio). The ethnic map of Celtiberia was highly localized however, composed of different tribes and nations from the 3rd century centered upon fortified oppida and representing a wide-ranging degree of local assimilation with the autochthonous cultures in a mixed Celtic and Iberian stock.