Some folklore about a cave under Craig Rhiwarth, recorded in 'Celtic Folklore - Welsh and Manx, by Rhys (1901). Cwm Glanhafan is on the mountain's eastern side.
Take for instance a cave in the part of Rhiwarth rock nearest to Cwm Llanhafan, in the neighbourhood of Llangynog in Montgomeryshire. Into that, according to Cyndelw in the Brython for [date missing on STA], p. 57, some men penetrated as far as the pound of candles lasted, with which they had provided themselves; but it appears to be tenanted by a hag who is always busily washing clothes in a brass pan.
According to Cofleing, this hill-top enclosure has several entrances in its tumbled stone walls, "running through precipitous crags", and inside are about 170 circular structures which one imagines were round houses (and some rectangular ones which were last used as a hafod (summer shelter) in the 19th century).
This small Bronze Age cairn sits inside the hillfort on a knoll at the summit of Craig Rhiwarth. It's made of scree stones and is about 1.5m across and 0.4m high, according to Coflein.
Obviously people were using the hilltop before the Iron Age fort was built: this is a small Bronze age cairn on the westernmost cliff edge, on the slope outside the hillfort. According to Coflein it's only 2m across and 0.4m high, and is made of small angular stones overlying a quarried out 'scoop'.