Next in importance to the roads were the fortifications. In addition
to those which Agricola had built (S26), either Hadrian or Severus
constructed a wall of solid masonry across the country from the shore
of the North Sea to the Irish Sea. This wall, which was about
seventy-five miles south of Agricola's work, was strengthened by a
deep ditch and a rampart of earth. (See map facing p. 14.)
It was furthur defended by square stone castles built at regular
intervals of one mile. Between them were stone watchtowers, used as
sentry boxes; while at every fourth mile there was a stone fort,
covering several acres and occupied by a large body of troops.
But the northern tribes were not the only ones to be guarded against;
bands of pirates prowled along the east and south coasts, burning,
plundering, and kidnaping. These marauders came from Denmark and the
adjacent countries (S37).
The Britons and Romans called them Saxons, a most significant name if
it refers to the stout sharp knives which made them a terror to every
land on which they set foot. To repel them, the Romans built a strong
chain of forts along the coast, extending from the Wash on the North
Sea to the Isle of Wight on the south. (See map facing p. 14.)
The greater part of these Roman walls, fortifications, and cities have
perished. But those which remain justify the statement that "outside
of England no such monuments exist of the power and military genius of
Rome."