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Quoting Bernard S. Bachrach’s and David S. Bachrach’s Warfare in Medieval Europe, c.400–c.1453, pg. 46:

In a manner very similar to their contemporaries in the provinces of Gaul and Germany, the Visigothic kings in what had been the Roman provinces of Hispania governed their realm from the fortress cities that they inherited from their imperial predecessors, including the Visigothic capital at Toledo, which was also the chief episcopal see of the Visigothic kingdom. These fortress cities, therefore, were the initial focus of the Muslim armies that crossed the Strait of Gibraltar in 711 under the command of Tariq ibn Ziyad.

Both Christian and Muslim accounts of the rapid and successful campaign by Tariq to overthrow Visigothic rule and install largely Berber garrisons in the cities of Spain point to the pivotal military rôle played by Jewish communities living in these fortifed urban centres.

For more than a century, the Visigothic kings had followed an inconsistent policy towards the Jews. As kings and would-be kings fought to control the royal throne, those who were supported by the Jews favoured them, and those who were opposed by the Jews persecuted them, including initiating efforts to confscate their property, and convert them to Christianity by force. This anti-Jewish policy was in force at the time of the Muslim conquest in 711.

As a result, the Jews, who were armed residents of the various fortress cities, as they also were in key cities east of the Pyrenees such as Arles and Narbonne, sided with the invaders. The Jews are depicted in the Arabic language sources as useful to the Muslim conquest and were regarded by later Christian writers, who also were opposed to the Jews in Spain, as the key element in making possible the Muslim conquest by giving them control over the fortress cities that served as the bases to establish control over almost the entire Visigothic kingdom.