From Imperial Spain: 1469-1716, by J. H. Elliott (Penguin, 2002), 2nd ed., Kindle Loc. 550-592:
The medieval Crown of Aragon, therefore, with its rich and energetic urban patriciate, was deeply influenced by its overseas commercial interests. It was imbued with a contractual concept of the relationship between king and subjects, which had been effectively realized in institutional form, and it was well experienced in the administration of empire. In all these respects it contrasted strikingly with medieval Castile. Where, in the early fourteenth century, the Crown of Aragon was cosmopolitan in outlook and predominantly mercantile in its inclinations, contemporary Castile tended to look inwards rather than outwards, and was oriented less towards trade than war. Fundamentally, Castile was a pastoral and nomadic society, whose habits and attitudes had been shaped by constant warfare – by the protracted process of the Reconquista, still awaiting completion long after it was finished in the Crown of Aragon.
The Reconquista was not one but many things. It was at once a crusade against the infidel, a succession of military expeditions in search of plunder, and a popular migration. All these three aspects of the Reconquista stamped themselves forcefully on the forms o Castilian life. In a holy war against Islam, the priests naturally enjoyed a privileged position. It was their task to arouse and sustain the fervour of the populace – to impress upon them their divinely appointed mission to free the country of the Moors. As a result, the Church possessed an especially powerful hold over the medieval Castile; and the particular brand of militant Christianity which it propagated was enshrined in the three Military Orders of Calatrava, Alcántara, and Santiago – three great creations of the twelfth century, combining at once military and religious ideals. But while the crusading ideal gave Castilian warriors their sense of participating in a holy mission as soldiers of the Faith, it could not eliminate the more mundane instincts which had inspired the earliest expeditions against the Arabs, and which were prompted by the thirst for booty. In those first campaigns, the Castilian noble confirmed to his own entire satisfaction that true wealth consisted essentially of booty and land. Moreover, his highest admiration came to be reserved for the military virtues of courage and honour. In this way was established the concept of the perfect hidalgo, as a man who lived for war, who could do the impossible through sheer physical courage and a constant effort of the will, who conducted his relations with others according to a strictly regulated code of honour, and who reserved his respect for the man who had won riches by force of arms rather than by the sweat of manual labour. This ideal of hidalguía was essentially aristocratic, but circumstances conspired to diffuse it throughout Castilian society, for the very character of the Reconquista as a southwards migration in the wake of the conquering armies encouraged a popular contempt for sedentary life and fixed wealth, and thus imbued the populace with ideals similar to those of the aristocracy.
The Reconquista therefore gave Castilian society a distinctive character in which militantly religious and aristocratic strains predominated. But it was equally important in determining the pattern of Castile’s economic life. Vast estates were consolidated in the south of Spain, and there grew up a small number of great urban centres like Córdoba and Seville, living off the wealth of the surrounding countryside. Above all, the Reconquista helped to ensure in Castile the triumph of a pastoral economy. In a country whose soil was hard and barren and where there was frequent danger of marauding raids, sheep-farming was a safer and more rewarding occupation than agriculture; and the reconquest of Estremadura and Andalusia opened up new possibilities for the migratory sheep industry of North Castile.
But the event which transformed the prospects of the Castilian sheep industry was the introduction into Andalusia from North Africa, around 1300, of the merino sheep – an event which either coincided with, or created, a vastly increased demand for Spanish wool. The Castilian economy during the fourteenth and fifteenth centuries steadily adapted itself to meet this demand. In 1273 the Castilian Crown, in its search for new revenues, had united in a single organization the various associations of sheepowners, and conferred upon it important privileges in return for financial contributions. This organization, which later became known as the Mesta, was entrusted with the supervision and control of the elaborate system whereby the great migratory flocks were moved across Spain from their summer pastures in the north to their winter pastures in the south, and then back again in the spring to the north.
The extraordinary development of the wool industry under the Mesta’s control had momentous consequences for the social, political, and economic life of Castile. It brought the Castilians into closer contact with the outer world, and particularly with Flanders, the most important market for their wools. This northern trade in turn stimulated commercial activity all along the Cantabrian coast, transforming the towns of north Castile, like Burgos, into important commercial centres, and promoting a notable expansion of the Cantabrian fleet. But during the fourteenth century and much of the fifteenth the full extent of the transformation which was being wrought in Castilian life by the European demand for wool was partially hidden by the more obviously dramatic transformations effected by the ravages of plague and war.
The Black Death of the fourteenth century, although less catastrophic in Castile than in the Crown of Aragon, provoked at least a temporary crisis of manpower, which may have helped to give the economy a further twist in the direction of sheep-farming.