Coflein lists this as a monument, classifying it as 'prehistoric', but gives no more information, disappointingly.
In a rock to the westward of Abergele, high above the road, after passing the large modern astellated mansion called Gwrych Castle, is a singular cavern termed Cefn Ogo, the entrance of which has not been inaptly compared to "the portal of a noble cathedral, arched, and divided within by what has the appearance of a great column." This cave seems never to have been thoroughly explored. It is said to be penetrable about forty yards, when further progress is arrested by a chasm or by water; I could not with certainty ascertain which. The entrance is dirty and unpromising, and the large stalactites with which it abounds are neither fanciful nor brilliant.
It is strange that no adventurous Welshman has yet penetrated the depths of this cavern, in defiance of the witch, who, according to local tradition, guards a vast treasure of gold at the very extremity of the cave. There is an absurd story told in the Month's Tour in North Wales, 1781, to the effect that four men, who attempted to explore the cavern, penetrated to a distance that required the consumption of three pounds of candles, and that two of the company were lost in its recesses.
Thomas Johnson, an enthusiastic botanist of the seventeenth century, visited this cave in 1639. The hill itself was, he says, called Garth Gogo, and the popular name of the cavern was Ogo Gumbyd, so styled after the giant Gumbyd, who was said to have been its original inhabitant. All traces of this tradition appear to be now lost. Johnson, who cared more for wild flowers than for old tales, notes having found in the cavern specimens of golden saxifrage and other plants.
p58 in 'Notes of Family Excursions in North Wales', by J. O. Halliwell, 1860. Online at Google Books