Two building platforms, and a possible third, are located on the north-east facing slopes of Gelligaer Common.
(a) SO1152402735: Platform defined by a terrace 17.5m long (north-east by south-west) by 6.8m wide, cut back into rising ground to create a hood 1m deep. The hood supports around its crest a bank 1.6m wide and 0.25m high, most evident on its west side but is partly obscured by a footpath across the top of the hood. The terrace projects above the slope to 0.75m high. Its surface is uneven and stony beneath the shallow turf.
SO1151202722: Immediately above (a) a grassy platform 2.5m (NE-SW) by 1.5m, hood 0.75m deep, terrace 0.5m high; its small size means that its status is uncertain but may have supported an ancillary structure.

(b) SO1153502727: on the south-east of (a), a grassy platform 13m long (NE-SW) by 5m wide, cut back to 1m deep and built out to 0.75m high. The lower end sides of the terrace are embanked to 2m wide and 0.3m high.

Additional sources:
OS record card: SO10SW17.
RCAHMW, Glamorgan Inventory vol. 3 part 2, p.30.
RCAHMW air photos: 94-CS 0780; 945106/52, 54.

David Leighton, RCAHMW, 10 May 2016