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.