text1: What tribes were the Romans fearful would invade from the North?
text2: The extent of Hadrian's Wall was 73 miles (117 km), spanning the width of Britain; the Wall incorporated the Vallum, a large rearward ditch with parallel mounds, and was constructed primarily for defence, to prevent unwanted immigration and the incursion of Pictish tribes from the north, not as a fighting line for a major invasion.