The Celts

Diachronic distribution of Celtic peoples:

  core Hallstatt territory, by the 6th century BC

  maximal Celtic expansion, by 275 BC

  Lusitanian area of Iberia where Celtic presence is uncertain

  the six Celtic nations which retained significant numbers of Celtic speakers into the Early Modern period

  areas where Celtic languages remain widely spoken today

The Celts (/ˈkɛlts/, occasionally /ˈsɛlts/) or Kelts were an ethnolinguistic group of tribal societies in Iron Age and Medieval Europe who spoke Celtic languages and had a similar culture, although the relationship between the ethnic, linguistic and cultural elements remains uncertain and controversial.
The earliest archaeological culture that may justifiably be considered Proto-Celtic is the Late Bronze Age Urnfield culture of Central Europe, which flourished from around 1200 BC. Their fully Celtic descendants in central Europe were the people of the Iron Age Hallstatt culture (c. 800–450 BC) named for the rich grave finds in Hallstatt, Austria. By the later La Tène period (c. 450 BC up to the Roman conquest), this Celtic culture had expanded by diffusion or migration to the British Isles (Insular Celts), France and The Low Countries (Gauls), Bohemia, Poland and much of Central Europe, the Iberian Peninsula (Celtiberians, Celtici and Gallaeci) and northern Italy (Golaseccans and Cisalpine Gauls) and, following the Gallic invasion of the Balkans in 279 BC, as far east as central Anatolia (Galatians).
The earliest undisputed direct examples of a Celtic language are the Lepontic inscriptions, beginning in the 6th century BC. Continental Celtic languages are attested almost exclusively through inscriptions and place-names. Insular Celtic is attested beginning around the 4th century through ogham inscriptions, although it was clearly being spoken much earlier. Celtic literary tradition begins with Old Irish texts around the 8th century. Coherent texts of Early Irish literature, such as the Táin Bó Cúailnge (The Cattle Raid of Cooley), survive in 12th-century recensions.
By the mid 1st millennium AD, with the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture and Insular Celtic had become restricted to Ireland, the western and northern parts of Great Britain (Wales, Scotland, and Cornwall), the Isle of Man, and Brittany. Between the 5th and 8th centuries, the Celtic-speaking communities in these Atlantic regions emerged as a reasonably cohesive cultural entity. They had a common linguistic, religious, and artistic heritage that distinguished them from the culture of the surrounding polities. By the 6th century, however, the Continental Celtic languages were no longer in wide use.
Insular Celtic culture diversified into that of the Gaels (Irish, Scottish and Manx) and the Brythonic Celts (Welsh, Cornish, and Bretons) of the medieval and modern periods. A modern "Celtic identity" was constructed as part of the Romanticist Celtic Revival in Great Britain, Ireland, and other European territories, such as Portugal and Spanish Galicia. Today, Irish, Scottish Gaelic, Welsh, and Breton are still spoken in parts of their historical territories, and Cornish and Manx are undergoing a revival.

Read more abut the Celts here.

The Wandsworth Shield-boss, in the "plastic" style, found in London