On this drawing two new red circles have been drawn, both centred at A.

 The smaller has a radius equal to half the y to A distance, and the larger equals the y to A distance.

The smaller circle appears to loosely position the Inner Ditch Radius, whilst the larger circle appears to similarly define the Counter Scarp.

Note: The outer rings appear to have a different centre to that of the Phase 3 centre, which coupled with the deterioration of the earthworks themselves may account for the inaccuracy of the Counter Scarp and Inner Ditch Radius when compared to other features of the plan.