Someone who is captured is technically considered a casualty

The Romans never fully colonised / Romanised Ireland, Wales, Scotland, Cornwall, Brittany and large parts of Iberia hence the survival of those Celtic languages through to modern times.

Samhain or Sauin was observed by Gaelic Celts throughout Ireland, Scotland and the Isle of Man. But Brittonic Celts from Brittany, Cornwall and Wales also held similar festivals - called Calan Gaeaf in Wales, Kalan Gwav in Cornwall and Kalan Goañv in Brittany.