Today it stands at the centre of Holyhead in the Welsh county of Anglesey.

Its date is unknown, but it is generally thought to be part of a late-4th-century scheme, associated with Segontium, which was used to defend the west coast against Irish sea-raiders.

The Romans also built a watch tower, within Mynydd y Twr on the top of Holyhead Mountain, which was almost certainly used as the fort's look-out point.

Both were possibly abandoned around 393, when the troops were sent to respond to the revolt of Eugenius of Gaul.

The Church of St Cybi still stands on the site today, with a small detached chapel (Eglwys y Bedd) that reputedly stands over Cybi's grave.