Picture a band of country two hundred miles deep and a thousand miles long, running from the Black Sea coast at Trebizond down through Cappadocia to the Cilician plain.
This is not a line. There is no line. There is a graded zone of mountain and high plateau, dotted with forts you can hold for a week and forts you can hold for a year, threaded by perhaps a dozen passes the Arabs can use to bring a real army through, and crossed at any given moment by raiders, drovers, smugglers, monks, pilgrims, deserters, and tax collectors, all of them carrying news.
The Roman system, in 838, had four moving parts.
The kleisoura. A fortified pass — the word means, literally, a closure. The eastern frontier has perhaps a dozen; the Cilician Gates above Tarsos is the most famous. A kleisoura was not a wall. It was a stone tower or two, a permanent garrison of perhaps thirty men, a stretch of trail you could not bring a horse up at speed, and a clear line of sight to the next watchtower and the beacon-line beyond.
The beacon-line. Nine fires from the frontier to the City. Each fire was a stack of dry brushwood on a known peak, kept lit-ready by a salaried watcher. When the first fire on Mount Argaios above Caesarea was lit, the second fire on Mount Samos saw it within an hour and was lit in turn, and so on across Anatolia to the lighthouse of the Pharos in Constantinople. The signal — Arabs through the pass — reached the emperor in something between four and twelve hours, depending on weather.
The akritai. The frontier soldiers. Smallholders with farms in the upland valleys, owing service in exchange for tax relief, organised under local officers who were often their own cousins. They were not a field army. They could not stop a real Arab column. They could harass it, count it, tell the empire what it was, and — most importantly — hide the population of their valley in the refuge caves before it arrived.
The refuge cities. The famous under-city of Derinkuyu and its forty smaller siblings: hollowed-out tuff cones with stable rooms, granaries, wells, churches, and millstone doors that closed from the inside. A village of three hundred souls could disappear underground for a fortnight on three days' notice, leaving their fields to be burned and their flocks to be driven off, and come back up when the column had passed.
Together, these four parts made a system that defeated, for two centuries, the wealthiest empire on earth. The Romans rarely won battles. They won the year. The Arabs would come up through the Cilician Gates in summer, ride about, burn some villages, find no army to fight, find no people to capture and ransom, and go home in autumn with less than they had set out with.
Why, then, was Amorium sacked in 838?
Because the system requires that someone — the emperor, the strategos of the Anatolikon theme, the local count — listen to the beacon. In 838 the emperor was at Dazimon, in the field, with the army the system was meant to summon. By the time he turned around, the Arab column had walked past every watchtower, every kleisoura, every refuge city, and was at the gates of Amorium with elephants.
The frontier did its job. Constantinople did not.
— Adapted from Children of New Rome: Amorium, Chapter 6.