http://stedmundsburychronicle.com/wstowtxt.htm

Background: The state of Britain after 400 AD
Since the 3rd century AD the east coast of Britain had been fortified by Rome. Branodunum (Brancaster in Norfolk), was one of the earliest forts, dated to the 230s. It was built to guard the Wash approaches. Gariannonum (Burgh Castle, Norfolk), was established between 260 and the mid-270s to guard the River Yare (or Gariannus Fluvius). It was garrisoned by the Equites Stablesiani Gariannoneses. Othona (Bradwell-on-Sea, Essex), was garrisoned by the Numerus Fortensium. Other east anglian forts were set up at Walton Castle, and the twin fort of Caister on Sea, standing on opposite banks of Yare to Burgh Castle. These were just part of a chain of forts around the south and east coast of Britain, mirrored by similar forts across the Channel. The British forts came under the control of the Count of the Saxon Shore at least by the late 4th century, and maybe earlier.