Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
Niger Prepares for War
[3.1.1]  The death of Pertinax, the killing of Julianus, the entrance of Severus into Rome, and his expedition against Niger have all been described in the preceding book. When Niger learned that Severus had occupied Rome, had been proclaimed emperor by the Senate, and was leading the entire army of Illyricum against him, supported by the rest of the infantry and a naval unit as well, he was greatly disturbed by these unexpected developments. He sent orders to the governors of the eastern provinces to keep a close guard on all the passes and harbors.
[3.1.2] He also sent word to the king of the Parthians [Vologases V], to the king of the Armenians, and to the king of the Hatrenians, asking for aid. The Armenian king replied that he would not form an alliance with anyone, but was ready to defend his own lands if Severus should attack him now. The Parthian king, on the other hand, said that he would order his governors to collect troops - the customary practice whenever it was necessary to raise an army, as they have no standing army and do not hire mercenaries.
[3.1.3] Barsemius, king of the Hatrenians, sent a contingent of native archers to aid Niger.
[3.1.4] The rest of his army Niger collected from the troops stationed in those areas. Most of the citizens of Antioch, especially the young men, who, in their instability, were enthusiastic supporters of Niger, offered themselves for military service, acting more in haste than in wisdom. Niger ordered that the narrow passes and cliffs of the Taurus Mountains should be defended by strong walls and fortifications, believing that an impassable mountain range would be a powerful protection for the highways of the East. The Taurus Mountains, which lie between Cappadocia and Cilicia, mark the dividing line between the East and the West.
[3.1.5] Niger also sent an army to occupy Byzantium in Thrace, at that time a large and prosperous city rich in men and money. Located at the narrowest part of the Propontic Gulf, Byzantium grew immensely wealthy from its marine revenues, both tolls and fish; the city owned much fertile land, too, and realized a very handsome profit from all these sources.
[3.1.6] Niger wished to have this city under his control because it was very strong, but especially because he hoped to be able to prevent any crossing from Europe and Asia by way of the Propontic Gulf. Byzantium was surrounded by a huge, strong wall of millstones cut to rectangular shape and fitted so skillfully that it was impossible to determine where the courses were joined; the entire wall appeared to be a single block of stone.
[3.1.7] Even now the surviving ruins of this wall are enough to make the viewer marvel both at the technical skill of the original builders and the might of those who finally destroyed it. Niger was thus acting, as he believed, with the greatest possible foresight to guarantee the utmost security.