The Saxon Shore forts were built by the Romans in the 3rd century AD as a defence against overseas raiders. In Norfolk the ruins of the forts built at Burgh Castle (Roman "Gariannonum"), guarding the estuary across from the island of Flegg, still exist, but there is now little remaining of the forts at Brancaster ("Branodunum") built on the north coast, and at Caister-on-Sea, on the east coast and near Burgh Castle. After the last of the armies of Rome had left in 410 AD most of the visible remains of Roman Britain slowly disappeared.