They didn't really need protection ~ they needed to be bathed!
Fortunately, and barely in time, Byzantine soapmakers arrived as refugees in Venice and other "hot spots" and civilization recovered ~ well, at least "chemistry" recovered. In the meantime French knights crushed the remaining Islamic outposts in Spain.
The rest is history.
If, of course, you ignore the Carolingian Renaissance. Don’t forget, it was a Frankish army under Charles Martel that defeated the Muslims at Tours. It was a combined Roman and Frankish army under Aetius that defeated the Huns at Chalons-sur-Marne. Western Europeans defeated the Vikings by converting them to Christianity (almost a third of the First Crusade was Viking) and they drove the Muslims from Spain.
Give the Medieval world another chance. By the Renaissance of the Twelfth Century Europe had developed a lively and vibrant culture, as seen in the explosion of Gothic architecture, learning and the universities. It ended badly with much misfortune, but hey, that coincided with the onset of the Little Ice Age. Cold is not good for humanity.