'(Arthur) ...was south-western, particularly Cornish. He was successor to a kingdom, sometimes independent, successor to a Cornish dynasty ..... First and last Arthur was Cornish; he was conceived at Tintagel and died at Camlann.' From 'Arthur, King in the West.'

The beacon chains across the South West, centred as they were on Lundy, were crucial to the security of Arthur's realm.

Alarms could be transmitted almost instantaneously.  Messages sent to ships. A mounted force dispatched.  The threat countered.

	Guinevere Guinevere - daughter  of King Leodegrance (Welsh Ogyr Van ),    wife of Arthur,    lover of Lancelot,    sister of Arianrhod.

A poem in the 'Book of Taliesin' - 'the Prieddeu Annwyn' - 'The Spoils of Annwn' contains the outline of a now lost legendary tale.  The poet tells of Arthur and his men sailing to the Fairy Fortress, Caer Sidi, aboard Arthur's ship Prydwen to free the captive Gwair ;-
"Perfect was the captivity of Gwair in Caer Sidi, Thanks to Pwyll and Pryderi's emissary. Before him no one entered in to it, The heavy, dark chain held the faithful youth. And before the spoils of Annwn sorely he sang, And thenceforth remains he till doom a bard. Three freights of Prydwen went we thither, But only seven returned from Caer Sidi."
The Fairy Fortress, Caer Sidi is, according to most experts,  Lundy island

Arthur had a navy. 

Geoffrey of Monmouth wrote that Arthur built or assembled a fleet of ships.

All three Dumnonian sub-kings Geraint, March or Mark, and Theoderic would seem to have been not only among Arthur's captains but were also seafarers. 

During this period there was continuing trade with the Mediterranean, with continental Europe, with Ireland;

all that would have to be protected as well as defending against invasion.

The Irish (Scots), the Saxons and the Picts, all the main enemies of the Britons, are recorded over and over as attacking by sea.  

In the south-west with its dependence on the sea the battles may have been fought on land, but frequently, one side arrived by sea.

The beacon chains, cavalry and ships would ALL be needed  to successfully fight off any sustained series of invasions/attacks by the Anglo Saxons.