In a previous post, I used an analysis of my Y chromosome DNA to trace the journey of my paternal ancestors from Y-chromosomal Adam, who lived in West Africa around 275,000 years ago (275 kya), to the founder of the R1b sub-haplogroup R-L23, who was born on the Caspian around 4,400 BC (6.4 kya). In this post, I continue my paternal ancestral journey to the most recently identified haplogroup founder: a Celtiberian living in what is now Portugal around 510 BCE. The sequence of Y SNPs defining my haplotree are shown in the following map. It summarizes this journey from R-L23 (4,400 BCE) to the most recent haplogroup founder (510 BCE), and continues on from the map published in the previous post.
There is an immense amount of recent research on European Bronze Age genetics and migrations and a very considerable unfinished debate on how to interpret the evidence. New evidence is being published almost weekly, as increasing numbers of ancient European remains are analysed and Y haplogroup identified. So this post will only aim to describe the big picture as best I can summarize it, and details and perhaps even some of the big picture will likely change in the future.
“They heard of the Great Barrows, and the green mounds, and the stone-rings upon the hills and in the hollows among the hills. Sheep were bleating in flocks. Green walls and white walls rose. There were fortresses on the heights.” 
As well as the barrows and stone-circles of my two previous posts, the Wessex Downs have quite a large number of Iron Age fortresses on the hilltops. The closest of these to Avebury, and the first I visited was Barbury Castle, a few miles south of Swindon.
Barbury Castle is perhaps the most remarkable of the Ridgeway hill forts. It’s on the edge of the Marlborough Downs with views in all directions, on the Ridgeway, and it’s huge! It is defined by a broadly elliptical double rampart with a ditch in between, which even after two millennia of erosion, remains quite steep and imposing. The fort was built in the 6th century BC as a refuge against warring tribes. At times of attack, people would bring their animals and shelter in huts inside the 12 acre enclosure. The outer bank was reinforced by huge sarsen stones and the inner bank was topped by chalk blocks and a continuous wooden fence.
The fortress is clearly visible in this satellite photo, taken from Google Earth Pro, and also just outside the western entrance can be seen a round barrow which dates from 1,700 BC.