I thought that the Celts were fairly civilised surely the Druids must have been as well? It sounds to me like there is some stereotyping contained here, from books movies tv series. The Celts and the Romans got along quite well, for a while at least, which would have meant that the Druids would have been left alone, at least until the end. At a guess, for several hundred years until the Romans changed their minds, and then Christianity arriving. In that time there was probably a lot of religious activity from both sides, so the Druids would probably have grown and be larger than we might think.