This is Wales' answer to Ireland's Newgrange tomb. Set on a small peninsula and visible from miles around, Barclodiad y Gawres is a reconstructed chamber tomb whose internal walls were decorated with zig-zags and spirals pecked into the stone. (Some of the stones are covered with algae and appear green.)
The tomb is kept locked and you have to pay a £5.00 deposit at the Wayside Shop in nearby Trefaelog for the key. They also hire out torches if you have forgotten to bring your own - and you will need one if you want to see the decorations. In the diagram on the left, which is not to scale and by no means a map, the black stones are the ones I found with decorations.
Unfortunately the decorations don't come out in the RealVideo so you'll just have to take my word that they are there - or, of course, go and see them for yourself.