Seems to be in the right spot for a trade port.
Layover on the way to Ireland and/or western and north western England and such.
Also, Cornwall’s north coast had tin mines.
There were places in the interior of western Britain that had very active trade links with the surviving parts of the Roman Empire, and the Severn estuary probably is the reason. In pre-Roman times the British Isles were known as the Tin Islands, so it wouldn’t necessarily be a surprise (if anyone ever cares to dig deep at Tintagel) that it was a port for a very long time.