The north-eastern frontier of the Roman Empire - one of the great gaps in modern knowledge of the ancient world - has long eluded research. It has defied systematic exploration and been insulated against all but passing survey by wars, instability, political sensitivities, language, and the region's wild, remote mountains, mostly accessible only on horseback or on foot. Its path lay across eastern Turkey, following the Euphrates valley northwards from Syria, through gorges and across great ranges, and passing over the Pontic Alps to reach the further shores of the Black Sea. Vespasian...