Almost twilight, and time for his watch to end. The stars were beginning to glimmer, and Silvius stood for a second watching them. His favourite was Orion, the hunter, and he looked for the three stars that made up his belt, before feeling for the studded leather one that held his own tunic together.

He was looking forward to going home, where a bowl of warm venison stew would be waiting for him. Then he would go to the small wooden room he shared with his younger brother Brutus, and his hard, straw-covered bed.

Turning to go, his eye was caught by movement in the distance.

At first, he thought it might be another merchant, rushing late towards the city, eager to get into the walls before the gates were barred. It was no good to be out after dark.

He looked again. The rider was moving fast, faster than any he’d ever seen before, even at the games they held for funerals. Was he running from something?

The horse’s hooves were kicking up a huge cloud of dust, pounding the dirt road, and in the dying light it was hard to make anything out. Silvius squinted.

The rider’s chest was bare, which was odd.

There was also something strange about the horse’s gait. It was maddened, clearly, but also totally in control, the rider seeming a part of it, to flow with it in a way that showed mastery.

Horse and rider came to the end of the straight section of paving that led to the city gates. The sun’s last rays gleamed off the horse’s chestnut flank.

A horn sounded nearby. Drusus, the boy on duty on the other side from Silvius, was making the alarm. A shout went down to the guards below. ‘Close the gates! Bar them! Stranger approaching!’

Silvius heard the hinges beginning to creak shut.

‘Let me in!’ came the rider’s voice, loud, foreign and deep. ‘Let me in!’

As the bright moon’s shining beams were beginning to spread over the landscape, Silvius realised that coming up the road at full gallop was a centaur.

How could it be? The centaurs were all meant to have gone, cantering away into the stars with the gods. Yet this was a fully grown male centaur. Fascinated, he watched the beast’s rippling movements.

‘Let me in!’ the centaur called again.

Sticking out of his flesh, in his right side, was an arrow.

Silvius gripped the top of the wooden palisade.

He was not meant to desert his post. That would mean a telling-off from Aeneas, or maybe even a whipping. After a momentary pang of indecision, he clattered down the stairs to the gates.

The two guards were already drawing them shut.

‘What are you doing? Let him in!’ Silvius hung over the banister.