I also used Fletcher as a reference for spellings of Spanish place names, which varied through the period covered here, and many sites had both Christian and Moorish names. I have used the term ‘al-Andalus’ to mean that part of the Iberian peninsula under the control of the Moors at any given time, and ‘Spain’ to mean the Iberian peninsula itself; the peninsula of course includes modern Spain and Portugal, both of which coalesced as political entities during the period up to 1492. Regarding names in England I have generally defaulted to modern versions. In all my choices I have aimed primarily for clarity.

The words ‘crusade’ and ‘crusader’, which I have used here for clarity, are relatively modern terms, derived from the twelfth-century term crucesignati (signed by the cross).

The historical ‘turning point’ at Poitiers, AD 732, was regarded by Edward Gibbon as ‘an encounter which would change the history of the world’. In his Decline and Fall of the Roman Empire (1776-1788), Gibbon opined that following an Arab victory, ‘Perhaps the interpretation of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mahomet.’ A more recent essay covering Poitiers is by Barry S. Strauss in What If? (Putnam’s, 1999); in the same volume Cecelia Holland speculates on the turning-back of the Mongols in 1242.

An account of the English opposition after the Norman Conquest is Peter Rex’s The English Resistance (Tempus, 2004).

The eleventh-century ‘flying monk’ Aethelmaer of Malmesbury (or Eilmer, Elmer or Oliver) was an historical character, mentioned in William of Malmesbury’s twelfth-century history Gesta Regum Anglorum (‘The History of the English Kings’). I have based some of Aethelmaer’s (fictitious) engine designs on the sketches of Leonardo da Vinci (1452- 1519), including the gunpowder engine; see for example Leonardo da Vinci’s Machines by Marco Cianchi (Becocci Editore, 1984). The career of the ninth-century Moorish aviator Ibn Firnas is described in, for example, the Dictionary of Scientific Biography (Scribner). An airport to the north of Baghdad is named after him, as is a crater on the Moon.

Roger Bacon, born c. 1220, was another historical character, a philosopher, educational reformer and an early proponent of experimental science. Bacon was indeed the first European to describe the properties of gunpowder, and he speculated on its use in weaponry. This was largely based on a study of Chinese firecrackers brought to him from the Mongol court by a missionary called William Ruysbroek, a few years after the date of Bacon’s fictitious encounter with the Engines of God shown here (Part II Chapter XXII). However (in our timeline) the manufacture of gunpowder remained unknown in the west for decades after Bacon, and the flintlock musket was not developed until c. 1550. See Brian Clegg’s The First Scientist: A Life of Roger Bacon (Constable, 2003), Gunpowder by Clive Ponting (Chatto & Windus, 2005), and Joseph Needham’s Science and Civilisation in China (Cambridge University Press, 1962).