I am working on a vague, under informed theory.

I've noticed in Yorkshire all of the hill forts seemed to fall out of use around 6-400BC. At that time Almondbury and possibly Wincobank were vitrified.

Yorkshire then seemed to have a lengthy period of relative peace. Until the Roman invasion when hill forts again came into use.

What if in the early first millenium BC a new religious political cult entered Britain? Impressing the locals by strange fire based ceremonies and secret rituals? And then creating a new political order, where sovereignty was assured but under the control of a new political/religious elite?

Like I say I'm very early in on this one, I tend to use theories to learn more about things and I usually disprove myself (or take a right turn in a new direction). I just think this theory may help us to date the coming of the Druids (if indeed they did come from elsewhere)