Taken from Coflein......
A large cairn, placed at an altitude of about 1,560 feet above Ordnance datum; it has been opened, and very completely destroyed in the process. Welsh historians of three generations ago were wont to associate the carneddau on the high land between Carno and Llanbrynmair, and especially the carnedd known as Twr Gwyn Mawr, with the conflicts mentioned in the Welsh chronicle called Brut y Tywysogion under the years 948 and 1080 A.D. In Archaeologia Cambrensis, 1853, II, iv, 8, the conjectures relative to the battle between Gruffudd ap Cynan (and Rhys ap Tewdwr) and Trahaiarn ap Caradog in the year 1080 are said to have been strengthened by the discovery of "javelin heads, battle-axes, and the infantry bills of that period" near the site of Twr Gwyn Mawr. In 1855 the cairn was opened by the Rev. David Davies, then vicar of Dylife. The remains of what appear to have been two separate interments were met with. In one, which was beneath three flagstones laid "on a level with the soil, a small leaf of bronze, about the size of a crown piece, but much thinner," was found. In another part of the cairn a cist 6 feet by 2 feet was unearthed, the floor "covered with black charcoal and ashes, intermixed with a profusion of small stones"; two flint arrow heads and a flint knife, having clear connection with this interment, were also discovered. The cairn is said to have measured 60 feet in diameter before the excavation. During that undertaking the interior was practically wholly removed, the stones being carelessly thrown out all round. Visited,12th July, 1910."
From the two ring cairns this huge and famous cairn can be seen lurking in the thin mist waiting patiently for it's turn. In good weather it would be seen from anywhere on the moor, alas for some good weather.
Like coflein states the interior is wholly destroyed, a right mess indeed, though I think I found at least one of the cist stones poking out of the rubble. But in this fowl weather I took some comfort in it's disemboweled interior, I had to lie down almost to get out of the wind, from inside, the wind started howling alarmingly I sat upright and it stopped, lied down and it came back got out and it went away, just normal wind noise and the constant whooosh whooosh whooosh.
I picked a careful route to the nearest turbine and climbed it's steps to take a view from twenty feet up, on a nice day this would have been a good place to survey the whole area, Three cairns (1 huge) two ring cairns a fallen menhir and an enclosure (of unknown date)
but only on a good day the wind that brings these giant whoooshing monsters almost threatens to fell them, as two of them creaked alarmingly as I walked under their whoooshing blades, that was more unnerving than the super strong wind that had such hampered getting up their in the first place.
Time now to get out of the wind a bit and detect those two small cairns down hill a bit to the east.