There are two separate earthworks here: Piercefield Wood Camp (the SW) and Piercefield Camp (the NE). Descriptions from Coflein:
Piercefield Wood Camp
A sub-rectangular enclosure, c.98m E-W by 82m, defined by a bank and ditch, except where it rests on precipetous slopes above the Wye valley, to the N. There is a possible S facing entrance.
Within the enclosure are features relating to the Piercefield ‘Wye walk’.
Piercefield Camp
A sub-rectangular enclosure, c.388m NE-SW by 94-116m, set upon the gently sloping summit of a steep sided promontory ridge above a bend in the Wye valley. Facing the neck of the ridge the enclosure is defined by double banks and ditches, elsewhere by a single bank, or ruined wall, above steep natural slopes.
The standing stone near the SW camp is apparently a landscaping feature associated with the now-ruined Piercefield Park. Coflein says:
Mid-late 18th century grotto and associated standing stone, part of Piercefield Wye Walk, set within the confines of a relict defended enclosure.
To the west of the grotto, next to the path, is a small standing stone, to the east of which is a large deep rectangular rock-cut hole (? natural).