This triumphal arch was previously an archway of   Caracalla's aqueduct, that carried it over the Via Appia in direction of the Baths of Caracalla. Only the central of three openings survives; it is decorated with the yellow columns. Later, it was used as the double door in the complex fortified courtyard, built for the strengthening of the defense. The arch bears the name of Drusus, a son of emperor Tiberius.