I've come up with a theory, its based on some sketchy info in places so feel free to shoot it down:

Vitrified Forts

Vitrification, with regard to Iron Age Hill Forts is a feature of some forts where all or part of the forts defensive rampart has been heated to a temperature such that the stone face of the rampart melts, to a lesser or greater extent. This produces a glassy appearance to the rock and in some cases has caused the rock to bond.

Vitrification, which seems to occur mainly in Scotland has two main causes:
1. Partial vitrification caused by accident or by invading enemies.
2. Total vitrification, caused by deliberate act by the builders/owners of the fort.

Partial vitrification can occur where an intense fire causes vitrifaction in one or more isolated locations and can be caused by accidental or natural causes, as in Almondbury. Or by deliberate act by an attacking tribe, where flaming bolts cause interior structures to burn uncontrolled.

Total vitrification probably only occured as part of a dedication ceremony at the building of the fort. And may have served as a warning to the attacking tribe and as part of the rousing of the blood lust of the tribe.

With a few exceptions vitrification occurs mainly in scotland, and therefore indicates a difference in customs north of the Forth.

Of the exceptions Wincobank in Yorkshire is worthy of mention.

Around 69AD Venutius overthrew the Brigantian Queen and became King of Brigantia.

As part of his defences against the coming Roman advance, he enlisted help from Scottish tribes, possibly as mercenaries. It is possible that Wincobank, known to be at his front line (but only by me at the moment) could have belonged to one of these Scottish tribes, which they dedicated to the battle in their customary manner.

I know its a bit early for theories like this, but it ties in with my research so I can attempt to disprove the key bits along the way