Borders of the Roman Empire, Frontier fortifications of the Roman Empire, a World Heritage Site across several countries

The borders of the Roman Empire combined natural barriers such as the Rhine and Danube with military roads, linked forts and, in places, continuous walls. In Britannia two walls were built one behind the other, Hadrian's Wall from 122 and the Antonine Wall between 142 and 144; in Syria and Arabia Petraea there was no continuous wall, only a net of border settlements and forts. In Dacia the Limes Moesiae joined two or three lines of vallum with a Great Camp and many minor camps.

The Latin term limes only acquired an administrative sense after the third century, when it denoted a military district under a dux limitis. In Britannia two walls were built one behind the other: Hadrian's Wall of 122, garrisoned by 50,000 soldiers, and the Antonine Wall of 142–144, abandoned by 164 and briefly reoccupied in 208 under Septimius Severus. In Dacia the Limes Moesiae combined two or three lines of vallum with a Great Camp and many minor camps.

The border zones shaped how European populations developed through trade routes, military posts, and places where Roman and non-Roman peoples met. These areas show how people from different worlds lived and worked alongside each other.

The frontier sections lie in many countries, from Hadrian's Wall in Britain to the Limes Tripolitanus in North Africa, so pick one stretch before travelling. At the Saalburg fort near Frankfurt the reconstructed garrison can be walked through. The stone footings in the photographs belong to a single excavated frontier site, where low walls outline the rooms.

The Latin noun limes meant a path or balk between fields, a boundary marker, or any road before scholars applied it to the frontier. After the third century it became an administrative term for a military district under a dux limitis. The word was not used by the Romans for the imperial frontier itself.