Although there never was a Celtic empire or civilization as such, there was a Celtic culture. It began in an Alpine valley (it evades me at the moment whether that is in modern Switzerland or Austria) and eventually spread north and west to the areas that you mention, as well as all the lands in between. After Rome conquered most of the Celtic lands, the remaining areas, being no longer culturally connected, went their own ways culturally for a few centuries, which is why you see such a pronounced difference in the modern areas that consider themselves of Celtic origin. As for North America, many of the native groups are not even genetically, let alone culturally, related for many millennia.
In any case it's only a game. They could have put in a civilization of ship wrecked interstellar travelers, who lost everything and had to work their way back up, and it would still play the same!