The early 6in and 25in Ordnance Survey maps show "Supposed Site of Battle of Rhiw-saeson Fought A.D. 873" within the enclosure of Caerau hillfort (nprn 93037).
Other sources suggest that the only authority for the battle (the "Gwentian Brut") has proved to be a forgery and hence the battle can be disregarded.
(OS 495 card: ST08SE1)
B.A.Malaws, RCAHMW, 05 September 2006.