Frontiers of the Roman Empire: The Lower Danube Limes in Bulgaria: :

The inextricability of the connection between the Roman limes and the lands it ran through is easily observed and there is even a regularity to be noted: some modern borders actually follow the line of the ancient defenses. At the same time there are significant differences between particular sections of the limes, resulting primarily from the topography, climate and changing strategic importance. The Bulgarian section of the limes is a perfect illustration. For a considerable distance it follows the Danube, which is at the same time the border between Bulgaria and Romania. The landform the location of major settlements at convenient river crossings, natural barriers separating the region from tribal territories and the relatively hospitable region of the Balkans all contributed to making the region north of the Haemus mons suitable for Roman urban and rural settlement, agriculture and overland commerce. The Danube was always of foremost importance. A major natural obstacle and at the same time a convenient communication route, it was easily defendable and facilitated control of trade routes, permitted quick and easy transport of goods, including army supplies, not to mention the movements of the army itself. The region was also relatively quickly Romanized, the process clearly stimulated by the presence of Roman troops transferred to the provinces mainly for the Dacian wars. Other factors of influence included civilians engaged in trade and agriculture, a rapid pacification of the local tribes and the productive coexistence of different ethnic groups, with a significant cultural input of the Greek colonies and poleis in the Black Sea littoral, which gave in effect a synthesis of different traditions, languages and cultures. Not the least in this process of cementing unity in the region was a common enemy the Goths, Ostrogoths, Huns, Karpai and Avars to mention just a few. See more