Excerpt from Tony Wilmott's BIRDOSWALD

Birdoswald/Banna Roman Fort

NOTE: The best estimation for the occupation of sub-Roman/early medieval Birdoswald is c. 420-520 A.D.  This is in perfect accord with proposed "reigns" for Uther and Arthur. I've theorized that the legendary star marking the beginning to Uther's rule was the comet of 442.  Arthur died c. 537 at nearby Camboglanna/Castlesteads. 

From Tony Wilmott's BIRDOSWALD Excavations of a Roman fort on Hadrian's wall

and its successor settlements: 1987-92 -

The existence of these structures and the identification
of these Periods are the most significant results of
the excavations. Nowhere else on Hadrian's Wall, or
indeed in northern Britain, has such definite evidence
of sub-Roman continuity been identified. This, of
course, means that the most fundamental questions of
date, status, function, and character must remain controversial.
An average chronology has been offered,
based on the potential for survival of the buildings.
This chronology suggests a lifetime of 50 years for each
of Periods 5 and 6, with a start date c 420. This brings
the end of Period 6 to c 520, though other chronologies
are possible, and it is entirely reasonable to suggest
a later terminal date, perhaps as late as the early seventh
century. Comparison of the Birdoswald evidence
with the scanty evidence for the fate of limitanei in the
late Roman west suggests that no general conclusion on
this matter is possible, but that different units, even
within the same province and on the same frontier will
have had different responses to the situation in which
they found themselves. At Birdoswald it is suggested
that the garrison remained as a self-perperuating social
unit, possibly dependent upon receiving supplies from
the hinterland which had hitherto delivered tax in kind
to the army. It is suggested that this was a symbiotic
relationship in which the suppliers of the fort's population
were offered armed protection in rerum. The architecrural
form of the large timber buildings recall the
halls of the sub-Roman and Anglo-Saxon periods. One
can, perhaps, see the former garrison emerging through
time as a social unit with a strong relationship with a
hereditary commander or 'head-man, who might take
on an identity similar to that of a petty king of the period.
Eventually it might not be possible to distinguish the
heirs of the limitanei of Birdoswald from the other
armed groups which inhabited sub-Roman Britain,
though it may be that they derived legitimacy from a
Roman heritage and possibly the use of Roman symbolism.
A comparison has been made between Birdoswald
and the reoccupied hillforts of the south-west and the
fortified sites of Scotland. It is quite possible that the
sub-Roman fortified sites which must have existed in
northern England are none other than the Roman forts
themselves. Evidence for the period is scanty because it
is, by its very nature, difficult to identify and recover. It
is also susceptible to later damage, and has not, in any
case, yet been generally sought. This must become a
prime focus for future work on the northern forts.
There is at least a possibility that the small-long
brooch recorded by Bruce does come from Birdoswald.
If so it may well be an artefact contemporary in use
with Period 6. The eighth-century ring-headed pin,
however, cannot be held to synchronise with any
known structural period on the site, and is a tantalising
indication that continuity of occupation may have run
on until later centuries in some other part of the fort.

***

At some point in the late fourth or early fifth century
the use of the south horreum was changed again,
and included the provision of a hearth at its western
end. High quality finds and a Theodosian coin were
dropped around the hearth. This final reuse of the
south horreum is associated with a small garrison,
which evolved through the late-Roman and early post Roman
periods into a community akin to those inhabiting
defended enclosures elsewhere in the north and
west of Britain. The nature of reuse echoes the form of
the halls built as the foci of such communities.
The eventual collapse of the horreum was followed
first by the construction of a timber building on the
robbed walls of the north horreum, including a relaid
stone floor, and then by a second timber building, sited
partly on the via principalis in order to emphasise its
relationship with the porta principalis sinistra. These
developments lasted possibly one century, and suggest
that the site was deserted by c 520.