The fort is one of Europe's only three-walled Roman forts. The fourth side fronted the sea and was probably the site of a quay. Its date is unknown, but it is generally thought to be part of a late-4th-century scheme, associated with Segontium, 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 almost certainly used as the fort's look-out point. Both were possibly abandoned around 393, when the troops were sent to respond to the revolt of Eugenius of Gaul.