The tranquillity of Bryn Euryn hill, 365 feet above sea level, belies a dramatic yet little known past. Iron and Dark Age strongholds, the medieval castle of a notable Welshman and a later fortified mansion have all been built here, and a Roman expeditionary force disappeared nearby. Bryn Euryn rises above the North Wales town of Colwyn Bay, dominating the old Welsh kingdom of Rhos. The ancient fortresses stood on its summit and the ruins of the fortified mansion are on its seaward slope. Historical records hint that Ednyfed had a castle here but where was it?
The site commands extensive views of the Conwy Valley and coast, and was once the location of a hillfort, which was initially excavated in the autumn of 1997 for the Gwynedd Archaeological Trust. The excavations revealed a "massive, well-built" wall formed from quarried limestone, believed to have once been 3 m (9.8 ft) high and to have been fronted by 3.5 m (11 ft) of rubble. There were no archaeological discoveries permitting a certain identification, but the pre-Norman layout led David Longley, the study's director, to connect it with the early-6th century king of Rhos Cynlas the Red on the basis of the "Bear Den" (Latin: receptaculi ursi) mentioned in Gildas's passage on him and the surviving local name of the area as Dinerth, "Fort Bear"
The hilltop was used again in World War II first as an observation post by the Home Guard; it was then used by the RAF as one of the first radar posts used to detect German bombers heading to Liverpool.[