It's definitely nothing to do with Stonehenge (the beginning of the iron age in Britain is a thousand years after the last phase of construction at Stonehenge). Nor was there a significant change of population with the introduction of 'Celtic' culture.
I haven't read it in depth, but I did scan the reports for possible explanations of the war grave interpretation. This is explicitly mentioned in the report, and is based on the 'awkward' positions of the bodies, just under and mixed with a layer of rubble. One skeleton (actually a few scraps of surviving bone) was a baby that was either very young indeed or unborn. On its own, 2 meters from a young adult, (probably female, but badly fragmented). I suppose they could be mother and new-born child, but not really a construction worker.
It is possible that this was an act of violence,but I doubt the slavery angle. If this was the result of winnowing out those who wouldn't make good slaves, why are there no elderly men and why are apparently healthy adult women in the grave? Women can do physical labour as more recent episodes in the history of slavery show.
Not all the men would have been fit to fight, and not all would have been captured. If the hillfort truly was over-run, I think the male bodies were simply segregated somewhere else.
The bodies do show a history of injury, but largely healed and nothing like a blow to the head is evident (admittedly, burying the bodies under rubble might have obscured this point).
One of the adults shows evidence of an unusual diet (tooth decay indicative of a sugary diet- common enough these days but not then) and a general absence of indications of a hard life. Privileged or sacred I wonder?
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