Caer Gybi, Roman fort in Holyhead, Wales.
Caer Gybi is a Roman fort in Holyhead featuring three thick defensive walls reinforced by corner towers and built using distinctive herringbone masonry patterns. The structure opens on one side directly to the natural harbor, creating an unusual defensive layout adapted to the coastal location.
Construction began around 300 AD during Roman occupation to defend the Welsh coast from Irish raids and maritime threats. After Roman withdrawal, the site lay dormant until medieval times when it became home to a religious community.
The medieval Church of St Cybi occupies the interior of the fort, marking where a monastic community took root in the 6th century. The site demonstrates how religious spaces later transformed ancient military structures into sacred ground.
The fort sits on the north side of Holyhead harbor and is easily reached from the town center with clear signage along Victoria Road. Much of the site remains above ground and walkable, though parts are embedded within the modern town and surrounded by later buildings.
This is one of only three Roman forts in Europe designed with three walls instead of four, an unusual choice born from practical necessity. The fourth side opens directly to the natural harbor, making the asymmetrical design an elegant solution to coastal defense requirements.
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