The end of the reign of Augustus (in 14AD) is a convenient marker for a shift in Roman strategic aims away from expansion and towards a “frontier maintenance”. The usual term for both the Roman frontier and the system of fortifications and garrisons which defended it is the limes (pronounced “lim-ees”), although this wasn’t the only word the Romans applied to it. I want to leave aside for a moment the endless, complex conversation about the degree to which the Romans can actually be said to have strategic aims, though for what it is worth I am one of those who contends that they did. We’re mostly interested here in Roman behavior on the frontiers, rather than their intent anyway.

What absolutely does begin happening during the reign of Augustus and subsequently is that the Roman legions, which had spent the previous three centuries on the move outside of Italy, begin to settle down more permanently on Rome’s new frontiers, particularly along the Rhine/Danube frontier facing Central and Eastern Europe and the Syrian frontier facing the Parthian Empire. That in turn meant that Roman legions (and their supporting auxiliary cohorts) now settled into permanent forts.

The forts themselves, at least in the first two centuries, provide a fairly remarkably example of institutional inertia. While legionary forts of this early period typically replaced the earthwork-and-stakes wall (the agger and vallum) with stone walls and towers and the tents of the camp with permanent barracks, the basic form of the fort: its playing-card shape, encircling defensive ditches (now very often two or three ditches in sequence) remain. Of particular note, these early imperial legionary forts generally still feature towers which do not project outward from the wall, a stone version of the observation towers of the old Roman marching camp. Precisely because these fortifications are in stone they are often very archaeologically visible and so we have a fairly good sense of Roman forts in this period. In short then, put in permanent positions, Roman armies first constructed permanent versions of their temporary marching camps.

And that broadly seems to fit with how the Romans expected to fight their wars on these frontiers. The general superiority of Roman arms in pitched battle (the fancy term here is “escalation dominance” – that escalating to large scale warfare favored the heavier Roman armies) meant that the Romans typically planned to meet enemy armies in battle, not sit back to withstand sieges (this was less true on Rome’s eastern frontier since the Parthians were peer competitors who could rumble with the Romans on more-or-less even terms; it is striking that the major centers in the East like Jerusalem or Antioch did not get rid of their city walls, whereas by contrast the breakdown of Roman order in the third century AD and subsequently leads to a flurry of wall-building in the west where it is clear many cities had neglected their defensive walls for quite a long time). Consequently, the legionary forts are more bases than fortresses and so their fortifications are still designed to resist sudden raids, not large-scale sieges.

They were also now designed to support much larger fortification systems, which now gives us a chance to talk about a different kind of fortification network: border walls. The most famous of these Roman walls of course is Hadrian’s Wall, a mostly (but not entirely) stone wall which cuts across northern England, built starting in 122. Hadrian’s Wall is unusual in being substantially made out of stone, but it was of-a-piece with various Roman frontier fortification systems. Crucially, the purpose of this wall (and this is a trait it shares with China’s Great Wall) was never to actually prevent movement over the border or to block large-scale assaults. Taking Hadrian’s wall, it was generally manned by something around three legions (notionally; often at least one of the legions in Britain was deployed further south); even with auxiliary troops nowhere near enough to actually manage a thick defense along the entire wall. Instead, the wall’s purpose is slowing down hostile groups and channeling non-hostile groups (merchants, migrants, traders, travelers) towards controlled points of entry (valuable especially because import/export taxes were a key source of state revenue), while also allowing the soldiers on the wall good observation positions to see these moving groups. You can tell the defense here wasn’t prohibitive in part because the main legionary fortresses aren’t generally on the wall, but rather further south, often substantially further south, which makes a lot of sense if the plan is to have enemies slowed (but not stopped) by the wall, while news of their approach outraces them to those legionary forts so that the legions can form up and meet those incursions in an open battle after they have breached the wall itself. Remember: the Romans expect (and get) a very, very high success rate in open battles, so it makes sense to try to force that kind of confrontation.

This emphasis on controlling and channeling, rather than prohibiting, entry is even more visible in the Roman frontier defenses in North Africa and on the Rhine/Danube frontier. In North Africa, the frontier defense system was structured around watch-posts and the fossatum Africae, a network of ditches (fossa) separating the province of Africa (mostly modern day Tunisia) from non-Roman territory to its south. It isn’t a single ditch, but rather a system of at least four major segments (and possibly more), with watch-towers and smaller forts in a line-of-sight network (so they can communicate); the ditch itself varies in width and depth but typically not much more than 6m wide and 3m deep. Such an obstruction is obviously not an prohibitive defense but the difficulty of crossing is going to tend to channel travelers and raids to the intentional crossings or alternately slow them down as they have to navigate the trench (a real problem here where raiders are likely to be mounted and so need to get their horses and/or camels across).

On the Rhine and the Danube, the defense of the limes, the Roman frontier, included a border wall (earthwork and wood, rather than stone like Hadrian’s wall), similarly supported by legions stationed to the rear, with road networks positioned; once again, the focus is on observing threats, slowing them down and channeling them so that the legions can engage them in the field. This is a system based around observe-channel-respond, rather than an effort to block advances completely.

Bret Devereaux, “Collections: Fortification, Part II: Romans Playing Cards”, A Collection of Unmitigated Pedantry, 2021-11-12.