The exact date of the fort's construction is unknown -- archaeologists give a range of late third to early fourth century. Britain had been Roman for two and a half centuries by then, and the empire was running into the troubles that would eventually pull it apart. Sea raiders from Ireland were attacking the west coast of Britain, slaving and looting in waves. Rome's answer along the Welsh and Cumbrian coasts was a series of small coastal forts -- outposts of the larger garrisons inland -- with high walls, watchtowers, and direct connection to a port. Caer Gybi was one of these. It was probably an outpost of Segontium, the much larger fort at modern Caernarfon. On top of nearby Holyhead Mountain, the Romans also built a watch tower at Mynydd y Twr, which functioned as the fort's lookout post -- the highest natural eye on Anglesey, watching the sea lanes from the Wicklow coast.

The fort is roughly rectangular. Three walls -- north, east, and south -- enclosed a small interior area. The fourth side, facing west, opened directly to the sea on what would then have been the inner edge of a sheltered cove. There was no need for a fourth wall when the wall would only have been broken by a sea gate; instead the line of the fort gave directly onto a quay where Roman naval patrols could tie up and resupply. At each of the three landward corners stood a circular watchtower, and the towers are the most surviving Roman military structures of their kind in Wales. Excavation outside the northern wall uncovered an adjoining building attached to the northwest tower, built in the same style as the tower itself and almost certainly also Roman -- possibly a small bath house, possibly a barrack annex, the function still uncertain.

Sometime in the sixth century, King Maelgwn Gwynedd -- a powerful and notorious ruler of the post-Roman Welsh kingdom of Gwynedd -- granted the old fort to Saint Cybi. Cybi, who had been born in Cornwall and travelled through Brittany and Ireland before settling in north Wales, founded a monastery inside the Roman walls. The walls became the precinct of a Celtic monastic community, and the church that grew up at the centre took the saint's name. St Cybi's Church is now medieval -- mostly fifteenth-century, with later additions -- but it stands on the same ground Cybi consecrated. Within the churchyard is a small detached chapel called Eglwys y Bedd -- the Church of the Grave -- which traditionally stands over the saint's burial place. The Welsh place-name for Holyhead is Caergybi, which means simply Cybi's Fort -- the town named after the saint, who named himself after the Roman walls.

The fort is now managed by Cadw, the Welsh heritage agency, and is freely accessible. The walls and towers stand inside the centre of modern Holyhead -- shops, houses, and roads have grown up around them. The southern wall is the most complete: high, faced with neat coursed stone, with a clear corner tower. The church inside is still active, holding services and welcoming visitors. In June 2024, an archaeological dig in the churchyard uncovered Roman and medieval material -- including building remains and fragments of pottery -- confirming continuous use of the site for sixteen centuries. Stand in the churchyard on a clear day and you can pick up the sea wind coming over the wall. The Romans who built this place could not see beyond the next decade; the church and the saint kept the walls in use for centuries longer than the empire that raised them.