Caer Gybi

The exact date of the fort's construction is unknown, but it is believed to have been built between the late 3rd-century or early 4th-century. It is thought to have been a smaller outpost of the larger Segontium fort, which was used to defend the west coast against Irish sea-raiders. The Romans also built a watch tower, within Mynydd y Twr on the top of Holyhead Mountain, which was possibly used as the fort's look-out point. In the 6th century, King Maelgwn Gwynedd gave the old fort to Saint Cybi, who founded a monastery there. The Church of St Cybi still stands on the site today, with a small detached chapel (Eglwys y Bedd) that reputedly stands over Cybi's grave.

The fort had three defensive walls and at the corner of each of the walls stood a circular watch tower, with the fourth side fronting the sea and was probably the site of a quay for the patrolling Roman warships. An excavation outside the northern walls uncovered an adjoining building to the fort's north-west tower, with a similar construction to the tower, and was likely to have also been Roman.