I am becoming more convinced that Sranwick was a trading post for cattle and other articles moving north-south in ancient times.
The "Scots Dyke" joining Richmond to Stanwick could have been easily broached by a raiding party but would have been a difficult hurdle when droving cattle. One would expect some sort of continuation from Sranwick to the River Tees at the lowest cattle-fordable point.
Consequently all cattle traded south would have to pass through Stanwick and the local ruler would no doubt raise a "fee" for the provision of the service.
This is possibly why there are no great quantity of "war artifacts" at Stanwick and the fact such a massive (700 acres enclosed) fortification has been built of relatively accessable terrain.
One would also expect that the enclosed area might by broken up into internal pounds or corrals probably by low stone walls to keep the various herds separate. These would have disappeared over the ages.
In this scenario it would be an annual cattle trading "fair" and would have had entertainments and many other goods traded.
Even before and after the compound existed this would have been a natural border place where such activities would take place.
This would all have stopped at the time of William the Conquerors harrying of the North. Subsequent political events would make it harder to continue.
I imagine that local traders used it but cattle could have come from as far afield as the Scottish Highlands. Some Highlanders may have made it this far however perhaps the Angles and their predecessors went into Southern Scotland to buy the cattle and bring them back through an area of their own influence. Consequently we might expect a similar trading enclosure inthe Falkirk/Stirling area and perhaps in the North Renfrew area between Paisley & Glasgow.
If the Angles moved into Strthclyde & Lothians trading they would have taken their language into the towns and they undoubtedly brought back the Celtic Church. Some obviously settled - it would not all be simply conquest.
Northumberland for long had more affinity with Scotland than England and it was only the incessant cross-border warfare that developed from the hardening English-Scottish nationalities that finally sealed the eventual division. In the time of Stanwick such division had not happened.
From my theory there obviously must have been a strong trading connection between the Renfrew area and the Tees.
From Stanwick the goods would have been on-sold to traders from the south.