The Parthians and the Romans were enemies engaged in ruthless and almost perpetual warfare, a life and death struggle which left few opportunities for peaceful contacts. The Parthian empire which the Romans regarded as the second world power was an oriental monarchy. Herodian stresses that the Romans were invincible on foot and the Parthians on horseback. Two commodities which enjoyed a reputation in Rome were "Parthian steel" and "Parthian leather". The Parthian tactics gradually became the standard method of warfare in the Roman empire. The ancient Persian tradition of large-scale hydraulic engineering was thus combined with the unique Roman experience in masonry. The Greco-Roman picture of the Persians as a nation of fierce and indomitable warriors contrasts strangely with another stereotype, the Persians as past masters of the art of refined living, of luxuriose vivere. The Persian influence on Roman religion would be enormous, were people allowed to call Mithraism a Persian religion.