Still, the Roman army, even with as many as 300,000 men, never had sufficient numbers to firmly guard such lengthy boundaries. As time went on, the Romans were increasingly forced to recruit the barbarians they had conquered into their army to sustain its numbers. These new warriors brought new methods, such as heavy cavalry and long swords, with them. While border fortifications provided bases and a fixed boundary, it was relatively easy for a determined barbarian horde to pierce Rome's long line. The solution was infantry that could march quickly on Roman roads to crisis points, or cavalry that could run raiders to ground. The analogy between the Petriana and the U.S. cavalry that patrolled the American West is obvious. Hadrian's Wall was not just a fixed fortification but a base for patrol.
While Petrianis may have been a real fort, I have moved it in my imagination from its likely flatland location near modern-day Carlisle (Uxello durum), where cavalry would have been most effective, to a more evocative but fictional setting of hill and river: a place that borrows from the geography of the Roman fort at today's Birdoswald, called Banna by the Romans, and that near Corbridge, called Onnum. Arden's fort of Tiranen is not based on any specific Celtic hill fort, but its design is typical of those found across Britain, and its described terrain is typical of the increasingly rugged Scottish countryside north of Hadrian's Wall.
The Scotti, incidentally, came-at the time of this story-from the isle that the Celts called Eiru and the Romans called Hibernia: modern-day Ireland. It was later that they gave their name to Scotland as part of that back-and-forth tide of conquest that ultimately made Great Britain the product of Celt, Viking, German, French, Irish, and Roman invaders, a melting pot of sword and blood. Thus a distant and ancient empire, Rome, helped seed a much later and even bigger one. Though Hadrian's Wall did not last forever, both the political boundaries it set and the elusive dream it represented- of permanent security, behind some kind of impregnable defense-remain with us to this day.