Feigning to be driven out of the city, he secretly joined Don Sancho, and offered to deliver the city into his hands if the king would only accompany him to a side gate.

Notwithstanding adverse omens, the credulous Sancho, believing him, rode off, only to meet his death at the postern gate, inside of which his murderer immediately took refuge.
On learning that his master has been slain, the Cid hastened to avenge him, and, as Sancho had left no heir, proclaimed Alfonso his successor.

We are told that this young prince had already heard of his brother's death through a message from his sister, and, fearing the Moors would not allow him to depart for good, had merely asked permission to visit his kin.

The wary Moorish king consented, but only on condition Alfonso would promise never to attack him or his sons, should he become king.
When Alfonso arrived at Zamorra, all the Spaniards readily did homage to him save the Cid, who refused to have anything to do with him until he had solemnly sworn he had no share in his brother's death.

To satisfy the Cid, therefore, Alfonso and twelve of his men took a threefold oath in the church of Burgos; but it is said Alfonso never forgave the humiliation which the Cid thus inflicted upon him.
The new monarch proved to be a wise ruler for the kingdoms of Leon, Castile, Gallicia, and Portugal.