There are neolithic burial mounds scattered all around Wales, but many have been completely stripped of their earthen covering by over-enthusiastic archaeologists and left as a stone shell. What makes Bryn Celli Ddu fascinating is that it's relatively intact; you can enter the barrow and pass into a stone-lined burial chamber that was used as a communal grave 5000 years ago.
To find it, follow the signpost off the A4080 down the country lane to the marked car park.