Over these road bodies of troops could march rapidly to any required
point. By them, and by similar roads, leading through France, Spain,
and Italy, officers of state, mounted on relays of fleet horses, could
pass from one end of the Empire to the other in a few days' time.
(See map below, and that facing p. 14.)

So skillfully and substantially were these highways constructed, that
modern engineers have been glad to adopt them as a basis for their
work. The four chief Roman roads[1] continue to be the foundation,
not only of numerous turnpikes in different parts of England, but also
of several of the great railway lines, especially those from London to
Chester and from London to York.

[1] The four chief roads were: (1) Watling Street; (2) Icknield
Street; (3) Irmin Street; and (4) The Fosse Way. (See map facing
p. 14.)

29. Roman Forts and Walls Defenses against Saxon Pirates.

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,