The leitmotif of this essay is the triumphal arch, with its central opening within the Ionic colonnade of the portico. The architectural element is a clear symbolic reference to ancient Rome. Starting from this evidence and in the light of the latest investigations, I outline here a chronological and semantic re-examination of the portico. The dating is to be ‘framed’ between the eighties of the 12th century and the early 13th century. In this period the history of Terracina is tied up with the authoritarian dominion of the Frangipani, an eminent aristocratic lineage of Rome, closely linked to the papacy. That they were among the protagonists of the construction of the portico is not to be excluded nor that the work was conceived following the III Lateran Council of 1179 and on the occasion of the solemn peace agreement that the Frangipani signed with the Terracinesi in 1185. The iconographic themes expressed in the mosaic frieze and in the sculptures can well be connected to this event. The theme of the triumphal arch continues to be at the centre of twentieth-century restoration work, of which unpublished materiali is here presented. Also unpublished is the group of drawings dedicated to the cathedral that the French architect Charles Percier created during his stay in Terracina in 1790. This is material of great interest, and not only for the portico, of which it philologically reconstructs the image of the triumphal arch, but as absolute evidence of Percier’s pioneering interest in medieval architecture.