The furthest records or surmises or inferences simply accept it as existing.
Some of these--guesses, let us call them--seem to show that there was some sort of structure there when the Romans came, therefore it must have been a place of importance in Druid times--if indeed that was the beginning.

Naturally the Romans accepted it, as they did everything of the kind that was, or might be, useful.

The change is shown or inferred in the name Castra.

It was the highest protected ground, and so naturally became the most important of their camps.

A study of the map will show you that it must have been a most important centre.