|SO042926||c.770 x 580 ft
(c.235 x 177 m)
This fort, also known to archaeologists as Caersws I, was discovered by Professor St. Joseph on aerial photographs in 1957 (published in JRS xlviii.97) lying partly within a loop of the River Severn, and was at first identified as a campaign camp or 'vexillation' fortress. Subsequent A.P.'s taken by Nash-Williams in 1969 confirmed the presence of a 'large fort' protected on at least two sides by extra defensive ditches. The enclosure measures about 770 feet from east to west by about 580 feet transversely, an area of around 10¼ acres (c.4.15 ha).
The foundation of the Llwyn-y-Brain fort has been attributed to the campaigns of Quintus Veranius in either 57AD or 58, and it was superceded by the smaller auxiliary fort at Caersws Village (Caersws II) about ¾ mile further west during the early campaigns of governor Gnaeus Julius Agricola c.78AD. This dating is based on evidence from the Caersws II fort, the fort at Llwyn-y-Brain having not yet been excavated.