In 929 AD, when Abd al-Rahman III declared himself Caliph of Córdoba, it was one of the western world’s greatest cities.
The population was rising towards half a million, exceeded only by those of Baghdad, Constantinople and perhaps Cairo. There were 70 libraries, holding perhaps half a million books – in the 12th century, Paris’s university had some 2,000.
At night, the city was lit by 35,000 streetlights, while London, with about 18,000 people, remained dark for another 700 years, as did the even smaller Paris.
Building on the hydraulic work of the Romans, the Arabs brought fresh water to their cities to irrigate what had
