Seems El Cid may have been similar to Drajo Mihilevic in Serbian lore, a great warrior known to fight for whatever side was good. Supposedly as opposed to EvilIt’s a nice myth, but he was a mercenary - his Islamic followers and masters gave him the name El CID (or al- Sayyid kind of “my lord” or “sir”)
Spain didn’t exist back then but to followers who spoke early Spanish he would have been ”El Campeador” (kind of champion/master of battle).
He fought for Muslims against christians and vice versa and by all accounts he was very good at it. The myth came after and was liberally enhanced by nationalist and far right, particularly by Franco when he was in power.
At the time the, the Muslims were the more educated, with famous scholars contributing to mathematics, astronomy, philosophy, optics, chemistry and medicine, and they put the maths into fledgling physics - christian Europe was largely illiterate apart from the religious elite and devoid of science.
Sorry to be pedantic and it might not interest people but I have family connections that make it interesting to me.
(plus a childhood family nickname of el caballero)
(plus family members who died battling the fascists in the Spanish Civil War)