The knights called themselves caballeros, and they were attended by hidalgos, lesser nobles. As they rode they joked, sang and drank. James thought they had a certain lazy arrogance. If they saw a stone wall standing they would run their horses at it to knock it down, if they saw a haystack they would torch it, if they saw a well they would throw stones down it to block it up, if they saw an irrigation channel they would dam it with dead cattle. With their crusaders’ crosses stitched to their sleeves they dedicated each new bit of destruction to Saint James the Moorslayer, and they dreamed of what they would do if they found a few plump Moorish women hiding out in the ruins. Thus they wrecked a landscape that had been intensively farmed for seven centuries.

Grace had no sympathy for James’s unease at all this. ‘You’re a hypocrite, ’ she said bluntly. ‘You gladly devote your pious, pointless little life to the development of devastating weapons. And yet you flinch when you see the results.’

James faced his conscience, and he knew she was right. He had had no choice about being assigned by his abbot to the engines project. But he was thirty years old now, and as the years had worn away he had become engaged in the intellectual exercise of the engines for its own challenge. It was thrilling to see these most remarkable toys emerge from heaps of wood and iron, saltpetre, sulphur and carbon - a thrill, his confessor warned him, that might be a compensation for other aspects of his life that he had piously put aside.

But he had, he realised, built a wall in his mind between the development of the engines and their ultimate purpose.

He said unhappily, ‘It’s just that I didn’t expect it to be like this.’ He waved a hand. ‘Is this war? This wanton destruction of property - there will be famine here in the winter - this savagery inflicted on the old and the ill, on women and children.’

She laughed at him. ‘What did you expect, chivalry? You ought to read a little more widely, brother. This is the way wars are fought now: French against Flemish, Italian against Italian, Moor against Christian . . . Why, we English pioneered the technique, in our long war with the French. You cut off your enemy at the knees by removing his food supply, by shocking his population into terror and submission. There’s even a word for it, I’m told: chevauchée. Wars are fought like this all over Europe now, like it or not.

‘So pray for the souls of the dead children, friar. But remember that the Pope himself says that a war for Christ is a just war, however it’s fought. And pray that you’re never on the losing side.’