There is an old fortification, on an insulated rock near Cwm y Glo, in this parish, called Caer Cwm y Glo; the wall is about nine feet thick, and in some places about six or seven high; the entrance is from the west; several other fortifications are visible from it, such as Dinas Dinorweg, Lys, and Pen y Gaer, in Llanddeiniolen parish; the circular watchtower, between the two lakes in Llan Beris parish and Dinas Dinlle, near the sea in Llan Dwrog parish.
From 'The Cambrian Register for 1795' in a section entitled 'a statistical account of the parish of Llanrug in Caernarvonshire.'
Substantial sounding remains? There's details from another visit from 1856 in Archaeologia Cambrensis and an empassioned plea for its protection and need to be valued here, as between 1854 and 1856 there had been much damage ("shamefully and needlessly maltreated").
There's no detailed information on Coflein though yet. Perhaps someone should take 'H.L.J.'s advice that "antiquaries should hasten to visit it before it is too late". It might be nice to check out whether the "small spring of water" still "trickles out from a rock near the single entrance on the south-west". With running water and natural fortifications it sounds quite the spot.