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.