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According to Professor Peter Salway in "Roman Britain" (and others), Celtic soldiers went into battle dressed only in trousers or pantaloons
This was remarkable to the Roman soldiers themselves who wore a tunic (that looked quite like a kilt) and were bare-legged.
(The kilt only became an institutionalised form of dress amongst the Scots in the early 1800s. Before that people wore plaid, one piece of cloth wrapped around the body and held with clasps.)
However, as the Romans noted, Scottish Celts often wore bare feet, even in the snow...