Man has been frequenting the summit and flanks of Mynydd Llysiau for, well, ever. This assertion is evidenced by nothing very obvious at all, but nevertheless the evidence is there. According to Coflein the peak has remnants of a field system with attendant hut circles.... and was also the location of a flint tool find, possibly mesolithic in date:
1) MYNYDD LLYSIAU FIELD SYSTEM - SO2015028050:
'Field system comprising at least 5 linear boulder banks running downslope on the western side of Mynydd Llysiau, with 2 other banks at right angles to 2 of the downslope banks. Associated with two Bronze Age hut circles'
2) MYNYDD LLYSIAU FLINT FIND - SO2045528555
Flint found by G Makepeace in 2000. Snapped flint blade, possibly Mesolithic
Unfortunately I wasn't aware of all this during my couple of visits here over the years, but have to admit that - like nearby Pen Twyn Glas - there is definately a serious 'sense of place' to be had here.
Anyone contemplating a visit may well wish to ascend via Rhiw Trumau [to the approx west] and thus take a look at the prehistoric remains upon Pen Trumau, too. Note that this beautiful route - affording exquisite views - begins near Castell Dinas hillfort, not to mention Cwmfforest, site of the Ty Isaf excavated long cairn.