What we know for certain is that, much later, a Roman settlement grew at Tai Cochion - a substantial trading town on the Anglesey shore directly opposite the Roman fort at Segontium on the mainland. Geophysical survey revealed something extraordinary: a planned street layout, 600 metres of typical Roman road eight metres wide running east toward the strait, flanked by perhaps 25 rectangular buildings in regular plots. This was not a military camp. It was not a defended Iron Age round-house settlement. It was not a villa. It looked, instead, like a small Roman town - the kind of civilian street grid you find at Sedgefield in Durham, transplanted impossibly to the remote western edge of the empire. Pottery shards dated from around 100 to 300 CE, including high-status second-century material. One burned-down building seems to have been a shop: many shattered Lezoux samian-ware beakers, the imported French pottery a Roman shopkeeper would have stocked, lay in the destruction layer.

The settlement makes sense if you read the geography honestly. Tai Cochion sits at what may have been the main crossing point of the Menai Strait - the place where men and cargo moved between the mainland fort and the conquered island. A trading town would naturally grow at such a crossing. To its west, the same survey found the outline of a much older defended Iron Age enclosure, sub-circular, 130 by 100 metres, ringed by a double bank and ditch. The probable Roman road runs immediately south of those defensive ditches. The pattern suggests the Romans found a pre-existing Iron Age trackway, improved it for their purposes, and let the Iron Age settlement empty out. The latest Roman coin found at the site was minted under Constantius II, between 337 and 347 CE - which puts the town's life span at roughly two and a half centuries before it, too, fell silent.

By the Middle Ages, the parish was part of the commote of Menai, in the cantref of Rhosyr - the same administrative division that included nearby Llys Rhosyr, the royal court of the Welsh princes. Centuries later, in the 18th century, a local man would change Anglesey again. Thomas Williams of Llanidan, born here in 1737, became the dominant figure in the global copper trade after the discovery of vast deposits at Parys Mountain in the north of the island. He was nicknamed the Copper King. He lived at Plas Llanidan, just down the lane from the old parish church. By the end of his career he controlled roughly half of Britain's copper output - sheathing for the Royal Navy's ships, coinage for the East India Company, currency for the slave trade. The wealth he generated reshaped Anglesey's economy. The hall where he lived is still here, a Grade II* listed building whose gardens are themselves listed on the Cadw register of historic parks.

Brynsiencyn, the small village at the parish's heart, has a post office, a shop, a kebab house, a pub, a primary school, a church, and a large chapel - the standard provisioning kit of a Welsh village that still works. Llanidan Stud breeds Welsh ponies of cob type, the section-C ponies and the larger section-D Welsh cobs - one of those breeds so identified with the country they cannot really be raised anywhere else. Near the village is the Anglesey Sea Zoo, where visitors can stand nose-to-glass with the same Menai Strait that the Romans crossed and the Druids defended and the kings of Gwynedd ruled. The water has been here all along. So have the field names.