After the Romans, things appear to have dropped into a relatively quiet period – at least in terms of remains still to be seen – during the times of Visigoth and Byzantine settlers.
Even the coming of the Moors, while their influence may be abundantly clear in the urban hearts and castles of many of Murcia's towns, seems not to have bequeathed anything notable in the Mazarrón area, with the exception of the Castillo de Calentín some miles to the north-west. It is not until well after the establishment of the Kingdom of Murcia in the middle of the 13th Century during the Christian Reconquest, that Mazarrón starts to emerge into the light once more.
Having been in the shadows of obscurity for over a millennium, Mazarrón, or las Casas de los Alumbres de Almazarrón, emerges into history once more in the late 15th Century. In May 1462, King Henry (Enrique) IV granted the right of exploitation of alum (alumbre) in the Mazarrón area, where it was abundant, to Juan Pacheco, the marqués de Villena. This individual then shared the rights with his cousin, the Governor of the Kingdom of Murcia, one Pedro Fajardo, the marqués de los Vélez. Alum had a number of uses – in medicine, in glass making, but above all, in the fixing of colours in the dying of cloth. For this reason, it was particularly used in the Low Countries and Britain. Towards the end of the 15th Century, it appears that the Casas de los Alumbres were effectively a large factory directed by Genoese merchants, (who had all the necessary contacts in the trade), for the extraction and production of the alum. At the end of the century, the social pecking order in the area would have had at its apex the administrators of the lands of the two marquises and their managers, followed by the Italian master workers responsible to the Genoese merchants who held the mine leases and finally the large number of ordinary workers. A byproduct of producing alum, was the waste red earth known as 'almagra' (or red ochre), which some years later would itself become quite valuable. With the alum industry, the town grew quickly, principally by the new fortifications and with two churches built very close to each other - that dedicated to San Antonio de Padua which was sponsored by the Marqués de los Vélez; and that dedicated to San Andrés and sponsored by the Marqués de Villena. The importance of the alum trade was immense and it is said that the Mazarrón industry was the second most important in Europe, after the mines of Tolfa in Italy, owned by the Pope. The rivalry between the two ended with the signing in 1530 of an agreement between Pope Clement VII and the Marqués of Villena, which divided the business in Europe. Spain (ie Mazarrón) would have the Portuguese, Flanders and British markets. The 16th Century saw a massive growth in Mazarrón alum production until, in 1568, King Philip II prohibited export to Flanders and, subsequently, to England.
There were, however, two other pressing issues in Mazarrón during the 16th Century. The first was the question of security. Pirates from North Africa were a constant threat. It is recounted that a major raid took place in November 1494, just before the beginning of the new century, when quantities of clothing, cattle, food and people were taken, including one Gonzalo Lechuga who, after being a captive in Algeria for some years, was finally returned to Mazarrón to retell his story, each time with increased heroic content. However, this clearly served him well as he was named the Mayor of Almazarrón by Lorca for several successive years at the beginning of the 16th Century.
The 1530's were an equally worrying time with repeated incursions taking place. Although many raids were seen off, the town was attacked in August 1539 when 23 boats landed on the coast unobserved. By far the most notable of the incursions, however, dates to 17th November 1585, now known and celebrated in Mazarrón as 'The Day Of The Miracle'. It was at dawn on 17th November 1585, that a watchman raised the alarm over strange noises that he heard. When people headed toward the sea they saw seven ships sailing away. On return, they were able to follow the footprints of the pirates along the side of the Sierra de las Moreras virtually to the town itself. The pirates had left weapons and other belongings in their flight, including the so-called 'Flag de los Moros' which now forms part of Mazarrón's heritage. When they reached Mazarrón, they were astonished to find that up to about 500 pirates had apparently got to within a few metres of the town undetected and had, equally inexplicably, then fled, also undetected.
It is now that the story begins to get even more interesting! An oil lamp in the main church, the Iglesia de la Purisíma, which had been unlit, was found the following day not only to be lit,but also dripping a considerable amount of oil onto a plate. Attention passed to the statue of the Virgin which had small drops of water on the forehead and larger drops on the right cheek. A priest wiped the statue's face, but the sweating continued. This continued for some time and was taken as a sign that God had spared Mazarrón from the pirates, also leading to the Virgin becoming the town's patron saint. Perhaps not surprisingly, something of a legend has also grown up around the events of that day during the succeeding centuries.
Whatever one may think of the stories of the miracle, it is undeniable that 16th Century Mazarrón had more than its fair share of troubles and dangers. A second problem with which it had to contend at this time was its relationship with Lorca. The population was growing quite rapidly as immigrants came from round about to profit from the work offered by alum extraction and production. By 1526, there was a population of around 160 households; perhaps 700 people. By now, the authority wielded over Mazarrón by Lorca was becoming increasingly resented. At times, the Council in Lorca appeared more concerned with obstruction than assisting Mazarrón's development and various edicts were issued preventing some activities, at least without a corresponding licence being issued. Similarly, it appears that the two Marquises progressively began to usurp the administration of justice from Lorca's hands. The scene was set for 'independence'!