As the son of Duke Peter of Cantabria, Alfonso held many lands in that region.

He is said to have married Ermesinda, daughter of Pelagius, who founded Asturias after the Battle of Covadonga in which he reversed the Moorish conquest of the region.

However, the few urban populations of these frontier regions fled to his northern dominions, leaving a depopulated buffer between the Christian and Muslim states.

Alfonso intended it this way; he wished to create an area in which any invading army would find survival difficult.

Besides the martial aspects, the demographic and cultural effects of this policy on later Asturian, Spanish and Portuguese history is large.