The Celts were a group of Indo-European peoples who, in their heyday (4th-3rd century BC), were settled in a large area of Europe, from the British Isles to the Danube basin, as well as some isolated settlements further south, the result of expansion towards the Iberian, Italic and Anatolian peninsulas.
United by their ethnic and cultural origins, by the sharing of the same Indo-European linguistic background and by the same religious vision, they remained always politically fractioned; among the various groups of Celtic populations there are the Britons, the Gauls, the Pannons, the Celtiberians and the Galatians, settled respectively in the British Isles, the Gauls, Pannonia, Iberia and Anatolia.
Bearers of an original and articulated culture, they were subject from the 2nd century B.C. to growing political, military and cultural pressure from two other Indo-European groups: the Germans, from the north, and the Romans, from the south. They were progressively subjugated and assimilated, so much so that already in late antiquity the use of their languages appeared to be in clear decline and their retreat as an autonomous people is testified by the marginalization of their language, soon confined only to the British Isles. There, in fact, after the great reshuffling of the early Middle Ages, the historical heirs of the Celts emerged: the populations of Ireland and of the western and northern fringes of Great Britain, speaking Brittonic or Goidelic languages, the two varieties of Celtic island languages.