There’s a gap of roughly a century and a half between the construction of the forts and the appearance of the name in the Notitia Dignitatum in the late fourth century, which we use to determine their function. The conclusion that the forts were built to keep out Saxons (or Germanic raiders in general) appears premature. Several other alternative reasons for the forts have been suggested.
When we look at the sources, we see that a commander, Carausius, was commissioned in AD 285/6 to defend the coasts of Belgica and Armorica (“to clear the sea”) against Franks and Saxons who “infested” those coasts (Eutropius Breviarium 9.21–22). Such raiders were active in the Channel, but it is not clear if Britain was also under attack. Carausius usurped power in Britain shortly after receiving his commission (see AW XIV.5). According to Aurelius Victor, Carausius was acclaimed imperator (Aurelius Victor Epitome de Caesaribus 39). We might interpret this as evidence he was deemed capable enough to defend the island against raiding hostile tribes, but Victor is not explicit. Eutropius and Victor (both possibly using the same source) are practically all we’ve got as far as written sources go. Thus, there is no clear evidence that raids were taking place on the British shores in the third century.
Stephen Johnson, in his influential (1979), is the main proponent of the most popular theory that the forts were indeed built to defend against Frankish and Saxon raids. Archaeology offers some support – the presence of coin hoards in eastern Britain, the appearance of city walls and evidence for large-scale destruction due to burning supports arguments for raids – but it is not
