Hadrian’s Wall (built about 122–25), which runs across northern England,
is typical of the Roman frontier defenses. A single masonry wall
with a concrete core and stone facings, the wall was seventy-three miles
long, between seven feet, six inches and nine feet, six inches thick, and
probably about fifteen feet high. In some places extensive earthworks
also survive. The wall follows the crest of hills, and it presented a sufficient
obstacle to encroachment by native people. Punctuated by towers
at mile intervals (popularly known as “mile castles”), the wall functioned
as a boundary and lookout post as well as a defense. Men walking or
standing on the top of the wall were protected by crenellations—raise
masonry panels (merlons) alternating with low sections (crenels) over
which soldiers could observe the wall or shoot at invaders. The wall’s
height and thickness provided protection against direct attack, but limited
sight lines from the wall-walk made any wall difficult to defend.
Sixteen forts housed the soldiers who patrolled the wall. Gates consisted
of a pair of doors flanked either by semiround (D-shaped) or square
towers. Such a wall was usually deemed to be sufficient protection. The
Aurelian wall around the city of Rome itself was a single wall and was
only strengthened in later years when the empire went into a decline.
World History
