From Wikipedia

The Celts

In various academic disciplines the Celts were considered a Central European Iron Age phenomenon, through the cultures of Hallstatt and La Tène. However, archaeological finds from the Hallstatt and La Tène cultures were rare in the Iberian Peninsula, and did not provide enough evidence for a cultural scenario comparable to that of Central Europe. It is considered equally difficult to maintain that the origin of the Peninsular Celts can be linked to the preceding Urnfield culture, leading to a more recent approach that introduces a 'proto-Celtic' substratum and a process of Celticization having its initial roots in the Bronze Age Bell Beaker culture.

The Iron Age Hallstatt (c. 800-475 BC) and La Tène (c. 500-50 BC) cultures are typically associated with Proto-Celtic and Celtic culture.

The La Tène culture developed and flourished during the late Iron Age (from 450 BC to the Roman conquest in the 1st century BC) in eastern France, Switzerland, Austria, southwest Germany, the Czech Republic, Slovakia and Hungary. It developed out of the Hallstatt culture without any definite cultural break, under the impetus of considerable Mediterranean influence from Greek, and later Etruscan civilizations. A shift of settlement centres took place in the 4th century.

The western La Tène culture corresponds to historical Celtic Gaul. Whether this means that the whole of La Tène culture can be attributed to a unified Celtic people is difficult to assess; archaeologists have repeatedly concluded that language, material culture, and political affiliation do not necessarily run parallel. Frey notes that in the 5th century, "burial customs in the Celtic world were not uniform; rather, localised groups had their own beliefs, which, in consequence, also gave rise to distinct artistic expressions". Thus, while the La Tène culture is certainly associated with the Gauls, the presence of La Tène artefacts may be due to cultural contact and does not imply the permanent presence of Celtic speakers.