Shortly after [the battle of] Sagrajas [in 1086 C.E.,] Rodrigo [Diaz, called El Cid] and [King Alfonso VI of Castile] were reconciled…. We do not know who took the initiative. What is fairly clear is that Rodrigo could make his own terms. The king was desperate, and was prepared—or could be brought—to pay Rodrigo handsomely for returning to his service. The author of the Historia Roderici tells us that
the king gave him the castle of Duáñez with its dependents, and the castle of Gormaz, and Ibia and Campóo and Eguña and Briviesca, and Langa which is in the western parts, together with all their territories and inhabitants.
He may have been quoting from a royal charter. The word ‘gave’ (dedit) is probably to be taken as meaning ‘entrusted the defence and/or administration of’ to Rodrigo. The king was not alienating chunks of territory but giving his vassal responsible and lucrative employment. Duáñez, Gormaz and Langa were important strong-points in the network of defences which guarded the Duero valley. Ibia, Campóo and Eguña were districts in the extreme north of Castile. Briviesca, to the north-east of Burgos, was contiguous to the Riojan territories of Count Garcia Ordóñez. This was not all. His biographer goes on thus:
Furthermore, King Alfonso gave him this concession and privilege in his kingdom, written and confirmed under seal, by which all the land or castles which he himself might acquire from the Saracens in the land of the Saracens should be absolutely his in full ownership, not only his but also his sons’ and daughters’ and all his descendants.
This remarkable concession has caused much perplexity to commentators…. [The author does] not see why the passage should not be accepted at its face value, as a summary of—possibly a quotation from—a royal charter granting to the Cid a very unusual gift. We should remember that the circumstances were unusual. Alfonso needed skilled commanders, and in the hard bargaining which was the essence of the lord-vassal relationship he held a weak hand. It was a vassal’s market. Rodrigo could call the tune.
— Richard Fletcher, The Quest for El Cid, chapter 10