Gariannonum, or Gariannum, was a Saxon Shore fort in Norfolk, England. The Notitia Dignitatum, a Roman Army “order of battle” from about AD 400, lists nine forts of the Saxon Shore in south and east England, among which one was called Gariannonor. It has been much discussed over the years in terms of spelling (Gariannonum, Garianonum, Gariannum), purpose (whether it really was intended for defence against Saxon raids), and location (Burgh Castle or Caister-on-Sea).
Gariannonum has usually been identified with Burgh Castle. However, modern reassessment of the Roman settlement 9 kilometres (5.6 mi) away at Caister-on-Sea has shown that it too had a military function. In Roman times, both sites lay on opposite sides of a large estuary (now 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. The military function of Caister-on-Sea is also open to doubt. Both sites probably operated together and one, or possibly even both, were known by the Romans as Gariannonum.1
(image by Ashley Dace, Creative Commons Attribution Share-alike license 2.0)