The Islamic conquest of the kingdom of Toledo brought about the disappearance of central authority in certain regions of the Iberian Peninsula. This is what happened on the Duero Plateau, which, between the eighth century and the mid-ninth century, was an area bereft of any type of complex political structure. The paper provides an analysis of certain elements of political organization during that period, defined by fragmentation and the existence of numerous small-sized territories that were associated with the management of common lands. It was in an area on the fringes of Asturians and Andalusians that a blurred frontier was drawn, where some influences of al-Andalus can be identified. After the second half of the ninth century, the kingdom of the Asturias spread across these territories at the same time as the county of Castile became consolidated. This increase in complexity created formerly non-existent struggles against the Muslims, and gave rise to a new frontier, although the areas south of the Duero generally remained outside the scope of Asturian, Castilian and Andalusian authority.