The fort is one of Europe's only three-walled Roman forts. The fourth side fronted the sea and probably the site of a quay. Its date is unknown, but it is generally thought to be part of a late 4th century scheme, associated with Segontium, which was set up 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 probably abandoned around 393 when the troops were probably sent to respond to the revolt of Eugenius of Gaul. In the 6th century, the old fort was given to Saint Cybi who founded a monastery there. The Church of St Cybi still stands on the site today, with a small detached chapel over Cybi's grave.
References
Arnold, Christopher J; Davies, Jeffrey L (2000). Roman & Early Medieval Wales. Stroud: Sutton Publishing.
