An ornate door inside St David's cathedral, Pembrokeshire, west Wales. St. David's Cathedral in Wales, one of Britain's oldest cathedrals, stands on the site of a 6th-century monastery founded by Dewi (David), a Celtic Christian monk. Considered the holiest site in Wales due to its relics of St. David, the cathedral was a major pilgrimage destination throughout the Middle Ages. It remains a thriving church today.