Almost 2000 years ago a heavily-fortified frontier ran right through Europe: the Limes. Warships secured the great rivers of the Rhine and Danube, forming the ""Water Limes"". Palisades and trenches, walls up to 4 metres in height with watchtowers and fortresses at regular intervals: the Limes ran right across the German territories, keeping the barbarians at bay. This gigantic barrier extended from Hadrian's Wall in northern England to the Black Sea. But what exactly was the Roman Empire so afraid of?
