Hadrian’s Wall was the centerpiece of a complex defensive system constructed by the Romans in the early 2nd century C.E. This defensive system began five to ten miles North of where the Wall with a line of Roman forts. These forts served as scouting and reconnaissance outposts, and provided advanced warning of any threats approaching the Wall. After passing these forts, an attacking army would have then encountered a deep ditch, which was often strewn with obstacles and sharpened stakes, directly in front of the curtain wall. The wall itself stood fifteen feet high, however the ditch in front of it forced attackers to climb over twenty-five feet in order to reach the parapet where the Roman soldiers stood. Even after overcoming the curtain wall itself, an attacking army was still faced with further obstacles in the form of a deep vallum – a wide ditch – flanked by two defensive earthwork parapets. This earthen defensive line ran parallel to the curtain wall along its entire length, and provided an advantage to Roman soldiers repelling attacks on the Southern side of the wall.
The Wall itself was 73 miles long from end to end, and incorporated eighty milecastles and seventeen forts along its length. Each of these mile castles held a small garrison of around sixty auxiliary troops, and housed a gate which allowed travel between the North and South. In between each of these milecastles were two watchtowers, which allowed Roman sentries to gain a commanding view of the surrounding terrain. The forts which were positioned along the wall each held about one-thousand troops, and were intended to allow the Roman army to respond to any serious threats along the wall quickly. To aid in this, a Roman road was constructed along the Southern side of the wall, and ran the entire length of the wall from coast to coast. This allowed soldiers to move up and down the wall rapidly in order to facilitate counterattacks if the wall was ever breached or scaled by an attacking force. This complex defensive design indicates that the Romans understood that the curtain wall was not an impregnable fortification, but instead served to blunt an enemy attack while simultaneously controlling peaceful contact between the North and South. The sheer number of gates which are present along the wall also indicate this.
The Wall in the North is of a far more simplistic design, however it is much more impressive in its size and scope. Standing 700 feet tall, 300 miles long and constructed of pure ice, the Wall is a purely defensive structure, and is not intended to facilitate easy movement between North and South. Unlike Hadrian’s Wall, the Wall in the North only had nineteen gates at its peak, however when Jon joined the Night’s Watch only three of these gates remained open, one at each of the three castles still manned by the Watch. The lack of passages through the Wall indicates what the structure is intended for: to safeguard Westeros from the Others and Wildlings who reside in the North. The nineteen castles which were constructed along the Southern side of the Wall also reinforce this purpose, as they do not have any defenses facing the South, and serve primarily as barracks to house the Night’s Watch.
 David J. Breeze and Brian Dobson, Hadrian’s Wall (London: Penguin Books Ltd., 1977), 31.
 Ibid., 32.
 Stephen Johnson, Hadrian’s Wall (London: The Bath Press, 1989), 19.
 Ibid., 30.
 H.H. Scullard, Roman Britain: Outpost of the Empire (London: Thames and Hudson Ltd., 1979), 62.
 Breeze and Dobson, Hadrian’s Wall, 37.
 George R.R. Martin, The World of Ice and Fire (New York: Bantam Books, 2014), 474.