:Caer Gybi (fort)

{{Short description|Roman fortlet remains in Anglesey, Wales}}

{{Use dmy dates|date=March 2025}}

{{Use British English|date=January 2017}}

{{Infobox military installation

|name = Caer Gybi

|image = Roman Wall around the site of St Cybi's Church - geograph.org.uk - 1414149.jpg

|caption = Roman Wall surrounding St Cybi's Church

|location = Anglesey, Wales, UK

|pushpin_map = Wales Anglesey

|pushpin_map_caption = Location in Anglesey

|gridref = {{gbmappingsmall|SH247826}}

|coordinates = {{Coord|53.3118|-4.6328|display=inline,title}}

}}

Caer Gybi was a small fortlet in Roman Wales in the Roman province of Britannia Superior. Its name in Latin is unknown. Today it stands at the centre of Holyhead in the Welsh county of Anglesey. Caergybi (or Holyhead) is a historic port town, named after the fort. The town's English name, Holyhead, is significantly different from its name in Welsh, as is the case with many coastal parts of Wales (see etymology).

The fort is one of Europe's only three-walled Roman forts. The fourth side fronted the sea and was 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 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.

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 (Eglwys y Bedd) that reputedly stands over Cybi's grave.