A mountain is a better protection than a river, but in 15 AD, when the Romans took over the Thracian lands between the Danube and the Stara Planina mountain, they had no choice: The mighty river, whose upper course they had already mastered, became the frontier of the expanding empire, setting a clear line between the civility of Pax Romana and the unruliness of the independent people on the other side of the river, the Barbarians, as the Romans called them.
The Danubian limes, or border, was one of the crucial fringes of the empire, a giant line of fortifications created to stop invasions from the north. The section of the river which is now in Bulgaria was fully incorporated into it. To guard their new border, the Romans created a set of castra, or military camps, castella, or small fortresses, and watchtowers along the river. Four legions were deployed in the camps along the Danubian shore of present-day Bulgaria: in Ratiaria, Oescus, Novae and Durostorum. An estimated 60,000 soldiers protected the porous river border, helped by a developed infrastructure net. A river fleet, probably stationed in the castellum of Sexaginta Prista (modern Ruse), patrolled this part of the river. Along the shore ran a military road ensuring secure connections. The Romans knew the importance of roads for military campaigns, and used to build these almost immediately after establishing their power over conquered lands. Sadly, modern-day Bulgaria lacks a similar riverside road along the Danube, and this leg of your Roman journey will regularly lead your far from the river and then back to it.
The remains of Ulpia Oescus
The Roman-built riverside infrastructure included also a bridge over the river at Oescus built in the early 4th Century.
The soldiers were not alone in the Roman realms along the river. The conquered Thracians were still there, and many of them lived in their old settlements close to the Roman camps and strongholds. And there were the civilians who would gather around each legion: merchants, craftsmen and publicans, prostitutes and soldiers' wives (legionaries were banned from marrying before the end of their 20-plus years of service; they would start families though not legal ones). This motley gathering of people from all corners of the empire would settle near the legion's camp, in civic settlements the Romans called canabae. After completing their service, many veterans would move to the canabae or would become landlords somewhere nearby.
Roman tower at Kula
By the beginning of the 2nd Century the military camps and their canabae had grown to such an extent that Emperor Trajan promoted some of them to the rank of colonia, recognising them as representatives of the imperial power. Being a colonia bore both prestige and practical gains, among them tax and judicial privileges. Happy with their new status, Ratiaria and Oescus added the emperor's family name, Ulpius, to theirs. In the 160s Emperor Marcus Aurelius went further, and promoted some lower Danube settlements to the rank of municipia, which granted them self-governance.
Marcus Aurelius's decision probably had something to do with the fact that after long decades of calm on the Danubian border (bar the Dacian attacks of 85/86 AD and the subsequent Dacian Wars), the enemy was finally at the border. In 170-171 the Costoboci ravaged the Balkan provinces of the empire.
Restored remains of Novae, near Svishtov
Eventually the border was pacified, but in the 3rd Century the so-called Barbarians used the opportunity to make the most of the economic and political crisis in the empire, and crossed the Danube several times, to disastrous effect. The tumult began in 239, and peaked with the invasion in 250 of the Goths led by Cniva, who wreaked havoc in the Danubian Plain, crossed the Stara Planina mountain and captured Philippopolis.
It was all downhill from then on. In 269 an estimated 300,000 Barbarians crossed the Danube, and in the early 270s Rome was forced to abandon the territories north of the river, which it had controlled since the times of Trajan. The settlements on the Danube became frontier outposts. At the end of the 3rd and in the early 4th centuries, an administrative reform, the change of the imperial capital to nearby Constantinople and controlled settlements of Goths in the Danubian Plain eased the pressure on the border for some time. This didn't last. In the 360s the Goths crossed the Danube again, reaching, in 378, as far as Hadrianopolis (today's Edirne, Turkey).
Many Bulgarian cities in the Danube started as Roman settlements and forts, like Tutrakan
In the 5th Century a new horror crossed the Danube: the Huns, led by a fearsome man, Attila. The Huns eventually continued westwards only to be replaced by another menace – the Avars, the Slavs and, later, the proto-Bulgarians. In the 6th Century the empire tried to ease the pressure on the Danubian border with military campaigns and reinforcement of fortifications, mainly under the emperors Justinian I (527-565) and Maurice (582-602).
But by the 580s the Avar destruction on the Danubian towns and forts had proved too much for the people there. Many forts and towns along the lower Danube were abandoned and the locals moved to the hills, in easier to defend fortresses.
The remains of the Roman border outposts, camps and cities dot the lower Danube today and are fascinating exploration sites. Sometimes they lie hidden in overgrowth or under buildings and streets from centuries of continuous inhabitation, sometimes they are exhibited and reconstructed.