I head across to the enclosure. Oblong in shape, the stonework of the walls still stands to a few courses high. Much more limestone lies around and about, so building material was certainly not an issue. What is rather less clear is why the structure has been built around a shake hole. I assume (geologists, please help) that the hole was already there when the walls were put up around it. It's not very big, so its mysterious portal-to-the-underworld qualities are fairly limited. Odd.
Stone built prehistoric enclosure below the slopes of Cefn yr Ystrad. Coflein description:
Small sub-rectangular enclosure, 7 x 8m with stone built banks up to 1m high, including boulders. No obvious entrance. In the centre is a sink-hole or cave opening. Located at the bottom of a steep hill, with many boulders on the ground.