Excavations have raised the possibility that the Roman legions arrived in the area as early as the first half of the reign of the Emperor Nero (A.D. 54-68).
The Roman occupation of south Wales began following the defeat of the war-like Silures tribe and the exiling of their great leader Caractacus to Rome. Their first fort, built on this strategically important site where the River Taff nears the Bristol Channel, was a campaign base constructed in still-hostile territory. Their defensive works surrounded a site even larger than the present ten-acres, on which were built their timber barracks, stores and workshops.
Around A.D. 75, when the Romans controlled the whole of Wales, the fort at Cardiff was rebuilt on a smaller scale and the workshops of craftsmen who gathered around any Roman base rose outside the new perimeter walls.
Around the year 300 AD, another new fort was built on the site with 10 foot thick stone walls backed by an earth bank. Its position gave new strategic importance as a naval base protecting the Empire against the menace of seaborne attacks by savage raiders from the west and north. This fort served until the Roman Army withdrew from the area in the early 5th century AD.
Little is known of the castle during the centuries that followed the Roman departure. Perhaps for long periods, the raiders from the sea made it untenable. The native princes of Glamorgan based themselves to the north and the ruined fort waited for a new army of occupation.