Often the question asked is 'who was Arthur?'. Much
more rarely asked is the question 'what was Arthur?'.
Where Arthur was from could be important, but
perhaps even more important is where he may have been based. Where
was his main centre of operations during his time as the 'battle
leader' of the Britons? (The term 'battle leader' was used in an
early poem that mentioned Arthur.)
In terms of the 'what', everyone who knows anything
about Arthur knows that he was 'King Arthur' - except that he wasn't.
A king was a Germanic title, 'se cyning', meaning 'the king',
a tribal leader (at first), becoming ever more grand as his territory
Romano-British official rank
Romanised Britons didn't have kings. During over
three centuries of Roman governance they had fully adopted the
Roman system of government. They had judges, or 'magistrates' in
their terminology, not kings (at least, in those areas which
remained governed by or which still acknowledged the country's
central authority). Among these would be a dominant magistrate,
one who was acknowledged by most or all as overlord.
Early in post-Roman Britain the magistrates would
have employed what we think of as generals who commanded the troops
in battle. The highest Roman title was Dux Britanniorum, and
there is no reason to assume that it was not still being used in
the fifth century. Also in Roman use at the time were military postings
with the title of comes, in essence meaning 'companion' and
usually translated as 'count'. This would have been the Roman title
used by whomever was in overall command of the Saxon Shore forts.
This title and function had been established by the Romans during
the fourth century. 
The Saxon Shore
As a way of examining the well-known list of
Arthur's battles below, it would be useful to establish a list of
the known Saxon Shore forts. They are Brancaster, Caister-on-Sea,
and Burgh Castle (all in Norfolk), Walton Castle (Suffolk, but lost
to coastal erosion), Bradwell-on-Sea (Essex), Reculver, Richborough,
Dover, and Lympne (all in Kent), Pevensey (East Sussex), and
ahead to Arthur's battles »
As can be seen, the Saxon Shore forts extended
from Hampshire in the south to The Wash, and possibly along the
north-eastern coast as well. The forts seemed to have been a
response to the appearance of seaborne Saxon raiders from the
mid-third century, with the defensive line they created apparently
being formalised in the fourth century.
The list of Arthur's battles points one towards the
understanding that at one point he was engaged in defending the
coastal areas - the shore. The Roman title for this was 'Comes
Litoris Saxonici per Britanniam' (the modern interpretation
normally says 'Count of the Saxon Shore for Britain').
With the Roman administration having been removed
in AD 409 (by whatever means - see the feature, The End of Roman
Britain, for a detailed examination) the Romano-British had to
organise their own defences. Early on there would have been infantry
on the Roman legion model, supported by cavalry. The Anglo-Saxon
Chronicle informs us rather brutally that they were not very
good, by boasting about the numbers of Britons dead.
By Arthur's time (towards the last decades of the
fifth century) these would probably no longer have been used as faux
legions, and instead the infantry would have consisted of guards
protecting the walls of forts, and guarding persons of importance,
such as those who died trying to protect the monks at the Battle of
Bangor-Is-Y-Coed in AD 613.
From the available evidence - a few scraps of
written material and a good deal of knowledge about the use of
fading Roman resources on the Continent - it can be concluded that
Arthur would have begun his military career as a cavalry officer
of noble birth (in Roman terms, he would have come from an equestrian
or patrician family). From there he rose to being responsible for
leading a form of rapid response force which would come to the aid
of a Saxon Shore fort and its guards (probably all or most of these
forts were towers).
There have been various estimates of the size of
his cavalry force, but around five hundred seems to be the most
reasonable figure quoted. Arthur would be under the orders of his
magistrate. Early in his career this might not have been the top
magistrate. But as success followed success he would have been
passed up the chain to fall under the command of the top magistrate,
with this individual effectively being a high king in tribal terms,
or an emperor in Roman terms.
Arthur's base of operations