Plan of the Caer Gybi fortlet
oriented with north at the top
(Adapted from Collingwood, fig.11)
The fortlet at Caer Gybi stood on the west shore of the natural harbour at Holyhead in Anglesey. The small church of St. Gybi stands on the site, within the defensive circuit of the fortlet, which consisted of three massive stone walls to the north, west and south, while the remaining eastern side was open to the harbour. Portions of this wall still survive over thirteen feet (4m) high in places. There were circular towers at three of the corners, the south-east corner tower at the far end of the harbour being omitted. Today only the north-west corner tower is wholly of Roman construction, the other two were rebuilt at a later period.
"Caer Gybi, on Holyhead, is an oblong enclosure of under an acre, with cylindrical bastions at the corners. It measures 230 by 150 feet; its walls are 5½ feet thick, partly in herring-bone masonry with putlog holes and bonding-courses of flat stones; they stand 13 feet high to the rampart-walk, above which are the remains of a parapet. The entrance was in the south wall (Segontium, 97-101)." (Collingwood, p.54)
The fortlet was built c.300AD to guard against Irish pirate raids, and was probably the base of several warships of the Classis Britannia, though no evidence has been found to confirm this supposition. Construction therefore took place during roughly the same time period as the Saxon Shore Forts were being built on the south and east coasts of Britain.