Two days later, when we came to the great River Clyd, I looked out across the plain towards Caer Alclyd and I knew the answer.

The Clyd valley forms a passage which cuts the northern wilderness east to west from Caer Alclyd at the Clyd estuary all the way to Caer Edyn. This vale also separates the hills of the south from the mountains of the north at the island's narrowest place. Anyone wishing to pass from one side of Britain to the other quickly must travel the Clyd valley.

Or, put another way: control the Clyd valley, and the whole of the north is yours. It is that simple. The barbarians knew this and they had been waiting for the spring flood at the Aberclydd to ebb so that they could lay siege to Caer Alclyd, the ancient fortress that guards the entrance to the passage to the east – as Caer Edyn guards it to the west.

We had forced them to act sooner than they might have done, that is all. They had not given up, and had no intention of leaving. Our appearance had not caused them to abandon their plan. What is more, gazing upon them as they were ranged about the caer, it became apparent that they had been joined by other hosts. Perhaps Angli had been hiding in glens and valleys all through the region, waiting to come together at this time and place.

Well, our numbers had increased, too. With Lot and his fifty, the Cymbrogi, and… I was struck by a sudden thought. 'Arthur – ' I said, turning suddenly to Arthur on my left, 'who is that in Caer Alclyd?'

'Do you not recognize the banner above the rampart?'

I squinted to gaze at the distant rock with its fortress on top. There was indeed a long banner hanging from a spearshaft fixed to the wall. It swung and fluttered in the wind, and I caught a glimpse of gold and blue. 'Bors?'

'None other.'

'Bors! What is he doing here?'

Arthur only shrugged. 'That we will have to ask him when we see him face to face. But it appears we must first clear these barbarians away from his gate so that we can talk.'

He made it sound as if it were but a moment's chore. God's truth, it was but the beginning of a work that would last the rest of the summer.

We met the enemy three times and three times defeated them. But they were determined, for they knew the importance of the fortress: whoever held it commanded the western half of the valley.

The first battle liberated Bors at Caer Alclyd. He had arrived from Benowyc only a day or two after Arthur had sailed north from Caer Melyn. So he had followed with his ships, thinking to meet us at the Clyd estuary. Upon coming into the river, however, he encountered the Angli host and had quickly sought refuge in the old fortress. The enemy then laid siege to it, and there the matter stayed.

This is how we found them: arrayed on the plain of the river, their camps ringing the great stronghold, or dun, as it is called in that region. Arthur gave orders for the glen to be blocked, and sent swift messengers south to Custennin in Celyddon, and to the lords of Rheged, bidding all to attend him. We settled down to wait until the British lords should arrive.

The lords of Rheged, my father included, joined us as soon as word came to them that Arthur was fighting in the region. Lord Ectorius, Cai's father, joined us from Caer Edyn. Custennin of Celyddon came with a warband of two hundred.

As soon as these last arrived, Arthur gathered the Cymbrogi together and led us in a prayer of victory. Myrddin held his hands above us in blessing, whereupon we pulled on our battle dress and mounted our horses. Then, taking our places at the head of the massed warbands, we left the glen and rode out onto the plain.

The charge was masterfully made. Long had Arthur observed the enemy encampment from our vantage of the glen. He knew how the battle lines would form, he knew – even before the barbarians knew it themselves – how they would respond to the charge. He knew it in his blood and in his bones.

Thus was that first battle short and sharp. Baldulf was beaten before he could mount a defence. Our a/a simply ran through them, and not once only: time and time again, charge upon charge. Great was the carnage, great the slaughter.

The flat plain was death to them. They could not stand against us. The siege broken, Bors swooped down from the rock fortress with his warband, sweeping all before him into the Clyd where many were drowned.

Seeing that his warriors could not fight us, Baldulf ordered the retreat, thinking to flee south to his ships. But Arthur had foreseen this, and our own footmen sealed the glen. In desperation the Angli and their minions fled to the north.

The barbarians were retreating to the forests of the lake region above the River Clyd, there to lose themselves in the dense and hidden pathways of those dark hills. Arthur called us to him while still on the battlefield.

'Cai, Bedwyr, Pelleas, Bors – assemble warbands and divide them among you. We will give chase.'

Idris and the other kings joined us, and up they spoke. 'Those forests are dangerous. The enemy can ambush us in there; they will lie in wait,' Idris complained.

Bedegran echoed his concern. 'Horses cannot manoeuvre in such thick woods. We would only do ourselves harm.'

Arthur could not quite hide his contempt. 'Since you fear, you will not be asked to undertake such dangerous duty. I have something else in mind for you.'

They did not like the way he scorned them, but it was their own fault. 'What is it that you require of us?' asked Maglos.

'You are to accompany Lord Ectorius and Myrddin back to Caer Edyn. I would have the shipyards protected and restored.'

'We are to become seamen?' sneered Idris. He thought it beneath him.

'Before this land is free, all my chieftains will be sailors. We will all fight as readily on the deck of a ship as from the back of a horse.' So saying, Arthur dismissed them to return with Myrddin and Ector, and we began the long and difficult task of running the barbarians to ground.

Idris and Bedegran had not overstated the danger, but had belittled the need. It had to be done: every barbarian who succeeded in eluding us would return to slay and burn again. They spurned Arthur's offer of peace, and had chosen the blade instead. Therefore we harried them mercilessly, allowing them neither rest nor respite. We pushed deeper and deeper into the wild hills driving the barbarians before us.

The hills north of the vale of Clyd are steep-sided and close set. The lakes are narrow, long, deep and cold: black-water realms ruled by keening eagles. Into these desolate hills we followed the enemy, pushing them further and further each day. And many days passed.

After many more days, we came to a place where a vast hump of land rises between two long lakes. The one is open to the sea and has no name; the other is called Lomond. A river called Dubglas joins them, running through a deep defile. And it was on this river that the barbarians chose to rally.

In this Baldulf showed wisdom. The cleft of the river was narrow, preventing a charge by the horses. And it sloped sharply up, giving the enemy the high ground they covet – if they cannot find a ford, a hill is best. And here they stood.

We attacked from below and the barbarians rushed down upon us. We fell back – as if overcome by their strength. Baldulf, eager to avenge himself for his defeats, pursued us. I still remember the gleam of their weapons in the hard sunlight as they plunged headlong down the scree-filled defile, screaming in triumphant rage. Those inhuman cries woke the stillness of the forest and made it quake. Down they rushed, with but one thought: to crush us utterly.

That was their mistake.

Arthur had held the second division in abeyance until Baldulf should commit himself. As the barbarians fell upon us, the hunting horn sounded and Pelleas, Cai, and Bors appeared up in the pass behind Baldulf. They had come round the hill and worked up the river pass from the opposite side.