Or you could just download Europa Barbarorum and play that[/shameless advertising]
I think that at this point both the Gauls and the British tribes still made use of chariots, and most warriors would carry javelins (much like any "non civilised" people)
Not long after the date you mentioned the Sennones (a cisalpine Gallic tribe) completely rolled over both the Etruscans and Rome in 387, who both still used the phalanx. I find it hard to believe that a disorganised horde could have done this. But I have to admit that I hardly know anything about Celts.
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