In all this business of borders and tolls and taifas, Robert was learning, to his surprise, that there was more than one Moorish country here in Spain. He had imagined that all of Islam would be united like one vast army, without individual faces or minds, under the orders of the caliph in Baghdad.

In fact the Muslims were diverse peoples. Even the armies who had originally invaded Gothic Spain three hundred years ago - whom the Christians called Moors, imagining they came from the old Roman province of Mauretania - had not all been Arabic. The leaders had been Arab, yes, but they had been outnumbered by their Berber warriors, who came from the harsh lands of the Maghrib just to the south across the Pillars of Hercules. Many of the Berbers’ descendants were prone to complain, even fifteen or twenty generations later, how they had been tricked by the Arabs when it came to parcelling up the old Gothic kingdom.

And, Robert learned, Muslims went to war with each other, as well as with Christians.

It had been fifty years since a single ruler, in Cordoba, had controlled all the Muslim lands in Spain. That ruler, remarkably, had been a second caliph, independent of the one in faraway Baghdad. ‘It is as if,’ Sihtric said, ‘a city like Paris or York hosted a second pope of its own.’

When the caliphate fell, al-Andalus splintered into taifas - so many of these little statelets that nobody had been able to count them; there may have been three dozen. But as is the way of politics and war the taifas had squabbled among themselves like fish in a pond, eating each other up until there were only half a dozen left.

As they drove steadily south the land became ever starker, drier, dustier, baked in the heat. And yet the irrigation channels brought green life to the land in great broad strips. Sihtric said that ‘water courts’ sitting in the towns supervised the upkeep of the irrigation systems, which were a communal treasure. The land actually seemed richer than England, where the peasants toiled with heavy ploughs. But then this land was not as God had designed it but as people had made it, people who had walked out of deserts, who knew how to extract life from the slightest drop of moisture.