The amazing civil engineering projects which the Romans completed in central Europe together form the Limes, a series of border fortifications, garrisons, walls, and barracks. Between the Rhine and Danube rivers, a sophisticated border structure was built.
But the purpose of these military constructions was, perhaps, not directly military. Instead of preparing for massive confrontations and battles, the goals of these efforts may have been economic and diplomatic. Historian Andrew Curry writes:
For centuries emperors used a mix of threats, deterrence, and outright bribery to secure peace. Rome negotiated constantly with tribes and kingdoms outside its frontier. Diplomacy created a buffer zone of client kings and loyal chieftains to insulate the border from hostile tribes farther afield. Favored tribes earned the right to cross the frontier at will; others could bring their goods to Roman markets only under armed guard.
A manned border would enable Roman officials to enforce the distinctions they had negotiated between various Germanic tribes. Vindolanda was a military installation just south of Hadrian’s Wall in England. Some of the men who staffed this location were from Germanic regions, or from other areas outside the empire.
Trade with northern Scotland or the northeastern Germanic tribes flourished at times during the Roman Empire, which last from 27 B.C. to 476 A.D.
Loyal allies were also rewarded with gifts, weapons, and military assistance and training. Friendly barbarians sometimes served in the Roman army; after 25 years, they retired as Roman citizens, free to settle anywhere in the empire. Vindolanda alone was home to units recruited from what are now northern Spain, France, Belgium, and the Netherlands. Iraqi bargemen once sailed England’s rivers under the banner of Rome, and Syrian archers watched over the bleak countryside.
Roman coins have been found in locations as diverse as Denmark, Sweden, or the northern Scottish island of North Uist. Germanic exports, like amber from the Baltic, were traded deep inside Roman imperial borders.
Individual Romans may have settled northeast of the Limes, outside the empire, and created agricultural estates for themselves. A settlement located in Hechingen-Stein, for example, is near the Limes. Whether it was inside or outside the empire might have varied: the exact borders changed over the decades, as aggressive or cautious policies correspondingly expanded or contracted the empire by a few miles.
Trade was also a foreign policy tool: The Roman-Germanic Commission in Frankfurt, part of the German Archaeological Institute, has a database of more than 10,000 Roman artifacts found beyond the limes. Weapons, coins, and goods like glass and pottery show up as far away as Norway and modern-day Russia.
It seems, then, that the borderlines of the empire were at least porous, and possible designed to not only allow trade, but to encourage it.
The Roman Empire, like the Roman Republic before it, had large amounts of foreign trade as an essential component of its economy. It cannot have escaped Roman bureaucrats that the borders should be managed in a way to encourage, regulate, and monitor imports and exports, and to collect tariffs and other taxes in the process.