Moel yr Eglwys ("bare summit of the church") is the highest point of Arenig Fawr. It's crowned by a large prehistoric kerbed cairn, but according to Coflein, the stones from this have been incorporated into a modern memorial and shelter.
Or take the following, from J. H. Roberts' essay, as given in Welsh in Edwards' Cymru for 1897, p. 190: it reminds one of an ordinary fairy tale, but it is not quite like any other which I happen to know:--In the western end of the Arennig Fawr there is a cave: in fact there are several caves there, and some of them are very large too; but there is one to which the finger of tradition points as an ancient abode of the Tylwyth Teg. About two generations ago, the shepherds of that country used to be enchanted by one of them called Mary, who was remarkable for her beauty. Many an effort was made to catch her or to meet her face to face, but without success, as she was too quick on her feet. She used to show herself day after day, and she might be seen, with her little harp, climbing the bare slopes of the mountain. In misty weather when the days were longest in summer, the music she made used to be wafted by the breeze to the ears of the love-sick shepherds. Many a time had the boys of the Filltir Gerrig heard sweet singing when passing the cave in the full light of day, but they were subject to some spell, so that they never ventured to enter. But the shepherd of Boch y Rhaiadr had a better view of the fairies one Allhallows night (ryw noson Galangaeaf) when returning home from a merry-making at Amnodd. On the sward in front of the cave what should he see but scores of the Tylwyth Teg singing and dancing! He never saw another assembly in his life so fair, and great was the trouble he had to resist being drawn into their circles.
From chapter 8 of 'Celtic Folklore, Welsh and Manx' by John Rhys (1901), which you can read at the Sacred Texts Archive.
At 2,802ft this is a pretty high one, it has to be said... the funerary cairn - or rather the remains of one - crowning the northern, and higher, of Arenig Fawr's two summits. According to Coflein:
'Remains of a large burial cairn on Moel yr Eglwys, the highest summit of the Arenig Fawr ridge. Stone built and circular in shape, the cairn measures circa 1.5m in height. The edge of the cairn is marked by a low kerb, except on the E side and the kerb measures a maximum of 0.3m tall and is 12m in diameter. The cairn material has spread beyond the original circumference and now measures circa 15m in diameter - the material has been remodelled to include an Ordnance Survey triangulation pillar, a World War II United States Air Force memorial and two walkers' shelters..'
Hmm... note the reference to the USAF memorial, which, like several other Welsh sites (e.g Cefn Yr Ystrad in the Brecon Beacons) adds an extra poignancy to a visit here, a sense that perhaps we are not so really different from our Bronze Age predecessors after all... we, like them, erect memorials to our heroes upon mountain tops. The gentlemen in question died here on 4th August 1943 when their B-17F 'Flying Fortress' hit the mountain while on a night cross-country training flight. Yeah, it was men like these, thousands of miles away from home, who played a major part in preserving the freedom we take for granted. Lest we forget, they were:
Lt James N. Pratt; Lt William A. Bowling; Lt Allen M. Boner; T/Sgt Frederic J. Royar;S/Sgt Walter J. Johnston; Sgt Walter B. Robinson; Sgt Phillip Simonte; Pfc Alfred B. Van Dyke.
If you decide to come and have a look for yourself, the classic route starts from approx SH846396 to the approx north and ascends via the impressive, if initially hidden, Llyn Arenig Fawr. As you would expect for such an isolated mountain, the views are exceptional when the summit is clear. But there's a lot more to Arenig Fawr than that, as the still remaining, occasional piece of aluminium debris will testify.