Complex sentence: The traditional position of the cathedra was in the apse, behind the high altar, which had been the position of the magistrate in the apse of the Roman basilica which provided the model type--and sometimes were adapted as the structures--for early Christian basilicas.
Split into simpler sentences: The traditional position of the cathedra was in the apse, behind the high altar. It had been the position of the magistrate in the apse of the Roman basilica which provided the model type--and sometimes were adapted as the structures--for early Christian basilicas.