Caer Gybi was a small fortlet

Castra

The Latin word castra, with its singular castrum, was used by the ancient Romans to mean buildings or plots of land reserved to or constructed for use as a military defensive position. The word appears in both Oscan and Umbrian as well as in Latin. It may have descended from Indo-European to Italic...

 in Roman Wales in the Roman province

Roman province

In Ancient Rome, a province was the basic, and, until the Tetrarchy , largest territorial and administrative unit of the empire's territorial possessions outside of Italy...

 of Britannia Superior

Britannia Superior

Britannia Superior was one of the provinces of Roman Britain created around 197 AD by Emperor Septimus Severus immediately after winning a civil war against Clodius Albinus, a war fought to determine who would be the next emperor. Albinus was the governor of Britannia during that civil war...

. Its name in Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 is unknown. Today it stands at the centre of Holyhead

Holyhead

Holyhead is the largest town in the county of Anglesey in the North Wales. It is also a major port adjacent to the Irish Sea serving Ireland....

 in the Welsh

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 county of Anglesey

Anglesey

Anglesey , also known by its Welsh name Ynys Môn , is an island and, as Isle of Anglesey, a county off the north west coast of Wales...

. Holyhead is named Caergybi in Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

, after the fort.

The fort is one of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

's only three-walled Roman

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

 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

Segontium

Segontium is a Roman fort for a Roman auxiliary force, located on the outskirts of Caernarfon in Gwynedd, north Wales.It probably takes its name from the nearby River Seiont, and may be related to the Segontiaci, a British tribe mentioned by Julius Caesar. The fort was founded by Agricola in 77 or...

, which was set up to defend the west coast against Irish

Irish people

The Irish people are an ethnic group who originate in Ireland, an island in northwestern Europe. Ireland has been populated for around 9,000 years , with the Irish people's earliest ancestors recorded having legends of being descended from groups such as the Nemedians, Fomorians, Fir Bolg, Tuatha...

 sea-raiders. The Romans also built a watch tower, within Mynydd y Twr on the top of Holyhead Mountain

Holyhead Mountain

Holyhead Mountain is the highest hill on Holy Island, Anglesey, and the highest in the county of Anglesey, north Wales. It lies about three kilometres west of the town of Holyhead, and slopes steeply down to the Irish Sea on two sides...

, 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

Monastery

Monastery denotes the building, or complex of buildings, that houses a room reserved for prayer as well as the domestic quarters and workplace of monastics, whether monks or nuns, and whether living in community or alone .Monasteries may vary greatly in size – a small dwelling accommodating only...

there. The Church of St Cybi still stands on the site today, with a small detached chapel over Cybi's grave.