One thing to think about- what is a 'celt'. More importantly, what would contemporary writers have considered a Celt-its not like Caesar was performing genetic and cultural studies of every 'celt' tribe he was wiping out. There certainly wasn't any unified celtic empire, like that map might lead people to believe.

All that means was that contemporaries called anyone living within that region a Celt- regardless of what language, government, religion, culture or technology each individual tribe, city or village may have had.