|This is a file from the Wikimedia Commons, Author: Herbert Ortner, License|
The first fortifications at Caernarfon were built by the Romans. Their military settlement (Castrum), which they called Segontsy, located on the outskirts of modern Carnarvon. Settlement was by the River Seiont, which flows into the Strait of Menai. The city's name is derived from the Carnarvon Roman settlement. Welsh town was known as «y gaer yn Arfon», which means "the stronghold in the land over against Môn"; Môn is the Welsh name of the island of Anglesey. Little is known about the fate of Segontium and its associated civilian settlement after the Romans departed from Britain in the early 5th century.