The so-called Puerta del Cordero, the southern access to the regal basilica of San Isidoro of Leon (Spain), could be considered as one of the earliest monumental portals of Romanesque European art. In the light of this possibility, this article deals with two main aspects. Firstly, there is a bibliographical summary of research that, between 19th and 20th centuries, studied the sculptures that decorate the portal, with the aim of claiming for some theories an authority that nowadays is played down and even forgotten by current historiography. Secondly, and more importantly, we furnish unpublished documents and a series of digital reconstructions of the medieval monument, putting forward different hypotheses concerning the image that the portal originally displayed, since it has undergone considerable reworking in modern times.