 In Roman times, both sites lay on opposite sides of a large estuary (the remnant of which is Breydon Water).The identification of Burgh Castle as "Gariannonum" is uncertain, and the name could apply to Caister-on-Sea.The name "Gariannonum" has been thought to derive from a Celtic root meaning "babbling river," which may refer to the River Yare at Burgh Castle, although the derivation is uncertain.