Durostorum in the context of "Bulgaria–Romania border"

Legio XI Claudia ("Claudius' Eleventh Legion") was a legion of the Imperial Roman army. The legion was levied by Julius Caesar for his campaign against the Nervii. XI Claudia dates back to the two legions (the other was the XIIth) recruited by Julius Caesar to invade Gallia in 58 BC, and it existed at least until the early 5th century, guarding lower Danube in Durostorum (modern Silistra, Bulgaria).

Aelius Catus was a Roman commander near the Danube who, according to Strabo's geography, transplanted 50,000 Getae from what is now Muntenia in Romania far to the south of Danube, in Moesia.

The Roman Empire had reached the Danube as early as 14 AD, when the commander Aelius Catus conducted an expedition beyond the river in order to keep away the restless Dacians and their new allies, the Sarmatians. But the legions deployed their troops only up to Durostorum, as modern northern Dobruja was left to the forces of the kings of the Sapaei, the allies of the Romans, helped by the forces commanded by a Praefectus orae maritimae (commander of the seashore).