From "Shropshire - An Archaeological Guide" -Michael Watson (2002 Shropshire Books):
"The hilltop had been a place of meaning to local Bronze Age communities long before the heights were fortified. Two ring cairns on the W hill summit are evdence for this. The first, disturbed and only partially surviving is surmounted by a modern OS trig. pillar. What remains of a central low mound is surrounded by the E arc of a stony ring bank, originally perhaps up to 28m in diameter. 80m to the SE, the other cairn is a low circular flat topped mound of earth, 23m across, with remains of a stone kerb around its edge. The excavation trench of 1932 is still visible and this located a 2.3m deep circular pit beneath the centre of the mound, but no direct evidence for the monument's date. The SW quadrant is truncated."
The Giant's Chair itself is a natural rock feature.