The Roman auxiliary fort at Coelbren, investigated by Colonel Llewelyn Morgan in 1904-7, was occupied, according to a pottery sequence, from the Flavian period (c. 74-84 AD) until the early to mid second century.
It is a rectangular earthwork enclosure, about 154m east-west by 142m, crowning a low knoll in otherwise marshy ground above the confluence of the Nant-y-bryn and the Camnant. There is an embanked annex on the east side.There appear to have been two phases of construction or occupation, and construction of the roads (Sarn Helen) leading south to Neath (NPRN 304620) and north to Brecon (NPRN 407122) are tentatively associated with the later phase. A marching camp (NPRN 301347) lies across the Canmant to the south-east. Cropmark features apparently representing a large building have been observed about 300m to the south of the fort alongside Sarn Helen.
Sources: Morgan in Archaeologia Cambrensis sixth series 7 (1907), 129-174
Simpson in Archaeologia Cambrensis 112 (1963), 13-76
Jarrett 'The Roman Frontier in Wales', 2nd edition (1969), 81-3
RCAHMW Glamorgan Inventory Volume I.2 (1976), 83-4
Sell in the Glamorgan-Gwent Archaeological Trust Annual Report for 1982-3, 65-6
Yates in Morgannwg 45 (2001), 84-6
J.Wiles, RCAHMW, 17 August 2004