If you aren’t familiar with Peter Connolly’s books, by all means make an effort to do so. They are supposed to be for children, but like many children’s books, they are much too good to be left to children! This particular book has a wonderful time line of Celtic helmet styles. I have redrawn several here to give you a better idea of the types of helmets used. There is no excuse for a good early Celt wearing a plain helm anymore!
400 BC Marne Area Bronze
300 BC Central France Bronze
200 BC Yugoslavia Iron
300 BC Senone’s Graves Iron and Bronze
helmet terminal with feather decoration
200 BC Alps Iron
10 AD Switzerland Iron
Source: Connolly, Peter. Hannibal and the Enemies of Rome.Macdonald Educational Ltd., 1978.