A neolithic Dolmen, this site has a wonderful fairy story attached to it, from Celtic Folklore, Welsh and Manx:
Our Pembrokeshire witness is a maiden Welshwoman, sixty years old.. she was born and has lived all her life within sight of the famous Pentre Evan Cromlech, in the home of her ancestors, which is so ancient that after six centuries of its known existence further record of it is lost.
[..she explained that:]..Spirits and fairies exist all round us, invisible. Fairies have no solid bodily substance. Their forms are of matter like ghostly bodies, and on this account they cannot be caught. In the twilight they are often seen, and on moonlight nights in summer. Only certain people can see fairies, and such people hold communication with them and have dealings with them, but it is difficult to get them to talk about fairies. My mother used to tell about seeing the "fair-folk" dancing in the fields near Cardigan; and other people have seen them round the cromlech up there on the hill (the Pentre Evan Cromlech).