It was a bright autumn day, and the northern British sky was a deep, rich blue. This was a sentry’s lookout point, here on the wall of the fortress of Brocolitia — in fact, strictly speaking Magnus was on sentry duty right now — and she could see across the countryside, to the farthest horizon in every direction. The land here was rolling moorland, bleak even at the height of midsummer, and as autumn drew in it was bleaker still. There was no sign of life save for a single thread of black smoke that rose toward the sky, far to the north, so far away its source was lost in the mist that lingered even now, so close to noon.

And if she looked to left or right, to east and west, she could see the line of the Wall itself, striding away across a natural ridge of hard, black rock.

The Wall was a curtain of tiled brick and concrete, everywhere at least five times a man’s height. A steep-sloped ditch ran along the north side. It was clogged with rubbish and weeds — and in some places the detritus of battles, broken sword blades and dented shields and smashed wheels; sometimes the hairy folk from the north would creep down to scavenge bits of iron. To the south, beyond the line of the road that ran parallel to the Wall, was another broad trench called the vallum. The vallum had been filled in here and there, to provide easier access between the fortresses on the Wall itself and the muddy little community of huts and roundhouses that had, over the generations, grown up to the south.

It was thrilling to think that the Wall’s great line was drawn right across the neck of the country. On a clear day she could see the sentries walking back and forth along its length, all the way to the horizon, like ants on a bit of string. And while on the north side there was nothing but moorland, heather, and garbage, on the south side there was a whole string of communities, inhabited by the soldiers and their families, and those who lived off them. It was like a single town, some of the soldiers said, a Thin Town eighty miles long, a belt of drinking and whoring and cockfighting and gambling, and other vices she understood even less.

But much had changed during the Wall’s long lifetime — so she had learned from Aetius’s dogged teaching. The threat the Wall faced had evolved. Compared to the scattered, disunited tribes faced by Hadrian who’d built the Wall in the first place, today’s great barbarian nations, like the Picts to the north of the Wall, were a much more formidable proposition.

Once, Aetius said, the Empire’s military might had been like the snail of a shell: break through it and you were into the soft, defenseless core of the settled provinces. After the disastrous barbarian incursions of the recent past, that lesson had been learned well. For all its imposing presence, today the Wall was only part of a deep defensive system. Far behind the line of the Wall there were forts in the Pennines and farther south, from where any barbarian incursion could be countered. And north of the Wall itself there were more forts — though few of them manned these days. More effective were the arcani who worked among the northern tribes, spies spreading dissension and rumor and bringing back information about possible threats.

Regina had grown to love the Wall. Of course it showed its age. Much of this old fortress had been demolished or abandoned, for much smaller units were stationed here now. And time had inevitably ravaged the great structure. Some of the repair work was visibly cruder than the fine work of earlier generations — in places the old stonework had even been patched up with turf and rubble. But the barbarians had always been pushed back, the Wall reoccupied, the damage by friend or foe repaired, and so it would always be. In the five years since Aetius had brought her here, enclosed by its massive stones, she had come to feel safe, protected by the Wall and the power and continuity it represented.

Conversely, though, she was prone to anxiety over the future. Overall there were far fewer soldiers in Britain than in the past, Aetius said: perhaps ten thousand now, compared to fifty thousand before the disastrous imperial adventure of Constantius, which had stripped Britain of its field troops. Two nights ago a red glow had been easily visible in the night sky to the east, and in the morning there was a great pall of smoke, coming from the direction of the next fortress to the east, Cilurnum. Troops had been dispatched there to find out what had happened, and hadn’t yet returned — or if they had, Aetius wasn’t saying so to her. Well, there was nothing she could do about that.

Regina shivered, and rubbed her arms to warm up. The Wall might be a safe place, but it was uncomfortable. The great masses of stone retained the cold all through the day. After five years here, though, she had gotten used to the brisk climate and needed nothing more to keep her warm but her thick woolen tunic. And she had learned never to complain about the rigors of life here, so stripped-down compared to life in the villa, which she still remembered brightly. She had no wish to be called a spoiled child again, even though she knew that as the granddaughter of the prefect she was given special privileges.

“… Ah,” Magnus said.

She walked back to him. “Don’t tell me you’ve moved at last, O Great General.”

“No. But your grandfather’s come out to play.” He pointed.

On the southern side of the Wall, Aetius had led his cohort out of the fortress and was drawing them up on parade. Aetius stood straight and tall, an example to his troops. But Regina understood how much effort that cost him, for at sixty-five years old he was plagued by arthritic pains.

The soldiers’ helmets and shields gleamed in the sun, and most of them wore the chill, expressionless bronze parade masks that had so terrified her when she first saw them. But their lines were ragged, with many gaps, and Aetius, waving his arms with exasperation, called out the names of the missing: “Marinus! Paternus! Andoc! Mavilodo! …”

Regina knew how infuriating Aetius found such ill discipline and lack of professionalism. Aetius had once served with the comitatensis forces, the highly mobile, well-equipped field army. Now he found himself the prefect of a cohort of the limitaneus, the static border army, and things were very different. These frontier troops had been on station here for generations. Indeed, nowadays most of them were drawn from the local people. According to Aetius, the limitaneus troops had become thoroughly indolent, even immoral. He raged at their habit of bringing actors, acrobats, and whores into the fortress itself, and their tendency to drink and even sleep when on watch.

All this was cause for concern, to say the least. Without a meaningful comitatensis in the country, these ragtag troops were all that stood between civilized Britain and the barbarians. And it was up to Aetius to hold them together.

Aetius consulted a clay tablet and called out a name. One unfortunate trooper stepped forward, a burly, harmless-looking man who didn’t look as if he could run a thousand paces, let alone fight off a barbarian horde.

“I was only drinking wine to wash down horehound to get rid of my cough, Prefect.”

“Do we not treat you well? Do you not enjoy medical attention even the citizens of Londinium would not be able to obtain? And is this how you repay us, by dereliction of duty?”

Regina knew that Aetius’s scolding was harder for the miscreant troopers to bear than the lashings that would follow. But now the fat soldier lifted up his arm and shook it, so his bronze purse rattled. “And is this how the Emperor repays me ? When was the last time you were paid, Prefect?”

Aetius drew himself up. “You are paid in kind. The temporary lack of coin—”