The fort itself was surrounded by an immense system of double ditches, and the streaming crowd had to cram itself onto a narrow causeway that approached the east gate. Ahead, thick walls with angular towers glowered down. The walls were built in the solid Roman fashion, with slave-worked concrete so strong it was said it would withstand the sea-coast weather for ever. But embedded in the walls Thalius identified fragments of broken columns, bits of statuary, even what looked like soldiers’ tombstones, all smashed and reused. Thalius wondered how many people here today knew that Claudius’s invasion force had once landed here, or mused on the irony that a triumphal arch commemorating that epochal landing had been demolished to build a fortress intended to repel new invaders. This was a grim age, an age of closure and huddling, not a time for grand gestures.

Still, regardless of its complicated history, today the fort was hosting Constantine himself, the Emperor of all the western provinces, ruler of half the known world. And on the Ocean beyond the shoulder of the fort walls Thalius glimpsed the purple sails of the ships that must have brought the Emperor and his retinue here. Thalius felt excitement grow inside him, a thrill he had barely known since he had been a child younger than Audax, waiting for the chariot races to begin in the circus outside Camulodunum.

As they passed through the fort’s west gate, Thalius and his party found themselves working through an access system mediated by officials from the local towns, the provincial government, the diocese of Britannia, even the prefecture of Gaul, and from the imperial court itself. All these officials, taking the chance to make a profit out of the Emperor’s visit, seemed to expect to have a coin or two stuffed in their hands for the favour of passing you through. The process was watched over by hard-faced members of the Emperor’s own German bodyguard - not the Praetorians, Constantine had run down those overpaid emperor-makers - who were not averse to a few hand-outs themselves. Tarcho grumbled as he handed over yet more coins from the heavy purse he carried.

But Thalius found it impossible to be sour, despite the queuing and the petty corruption. You couldn’t ignore the eagerness and anxiety, the hopes and dreams of the supplicants, for today Rome was here, on this windy British shore.

At last, thanks to his note from Ulpius Cornelius, Thalius found himself part of a crowd of petitioners drawn up before a stage, hastily erected just off the road inside the fort’s western gate - a stage on which Constantine himself sat, advisers and guards at his shoulders, patiently listening to complaints and pleas.