From the Llanrhystud Heritage Trail leaflet:
From here you will see Castel Bach and Castel Mawr (sic), the sites of two Iron Age hillforts facing each other. Local legend suggests that there was once a terrible battle between them and the gully dividing them is know as Pantglas (Pantgalanas) the dell of slaughter. However there was once a medieval castle overlooking the village to the northeast known as Caer Penrhos. The castle was thought to have been built by Cadwaladr ap Gruffudd between 1147 – 1149. There was much turbulence at the time and records show much bloodshed which might be associated with the legend.