Caer Gybi is a stoutly walled rectangular enclosure that crowned a low cliff overlooking Holyhead harbour. It is thought to be a late Roman (third or fourth century AD) strongpoint presumably concerned with maritime activities. At some point in the medieval period a monastery was established within the walls and this later became a large collegiate church (NPRN 27500). There is also a second medieval church or chapel, Eglwys-y-Bedd (NPRN 43590). In 1646 the Fort of Holyhead held a Parliamentary garrison.

It is a near rectangular enclosure, about 67m north-north-east to south-south-west by 44m. The walls are 1.8m thick and rise from 2.6m (internally) to 4.0m (externally) to a wall walk and ruined parapet. The eastern side is now marked by a more recent wall along the top of the old cliff, and the fort may once have been open on this, the seaward, side. There are round towers at each corner. Those at the western angles are about 5.0m across and appear to be original. The eastern towers are about 7.5m across. The north-east tower is thought to be a medieval rebuild, restored in the seventeenth century and late nineteenth century. The south-east tower was rebuilt in the late nineteenth century. The modern twin arched gateway in the south wall occupies an original entrance.

Excavations beyond the north walls uncovered the wall of a building abutting the north-west tower. This was of similar construction to the tower and is likely to have been Roman. There was no trace of a ditch on this side. Given that the work was refortified in the medieval period, the placement of a garrison here in the seventeenth century could reflect an earlier practice.

Sources: RCAHMW 1937 Anglesey Inventory (1937), 31-4
Griffiths in Archaeologia Cambrensis 103 (1954), 113-6
Nash Williams 'The Roman Frontier in Wales' (1954), 99
Jarrett 'The Roman Frontier in Wales' revised edition (1969) 136-7
Gruffydd in Archaeology in Wales 32 (1992), 76-7 - with plan.

John Wiles, RCAHMW, 13 July 2007