The paper aims to enlight the origin of a defence structure dated to the high Middle Age, originally located near the actual population of the city of Sant’Antioco (Sardinia, Italy) and actually gone. By a study of the archaeological, historical witnesses and the comparison between the architectural peculiarities of the castrum and the survived traces of its homologous structures in Africa, the paper tries to reconstruct what would have been the causes for its construction, proposing to identify them in the conflict between Byzantines and Moors after the Byzantine conquest of Africa during the first half of the VI century A.D.