The wall, (Scildweall or Bordweall in Old English, Skjaldborg in Old Norse) is a military tactic that was common in many cultures in the Pre-Early Modern warfare age. There were many slight variations of this tactic among these cultures, but in general, a shield wall was a "wall of shields" formed by soldiers standing in formation shoulder to shoulder, holding their shields so that they abut or overlap. Each man benefits from the protection of his neighbour's shield, usually the man to his right, as well as his own.
This tactic was used by many ancient armies including the Persian Sparabara, Greek hoplite phalanx formation, and Roman legions.
The shield wall came into use in ancient Greece during the late eighth or early seventh century BC. The soldiers in these shield wall formations were called hoplites, so named for their heavy weaponry (hopla, "ὅπλα"). These were three-foot shields made from wood and covered in metal. Instead of fighting individual battles in large skirmishes, hoplites fought as cohesive units in this tight formation with their shields pushing forward against the man in front (to use weight of numbers).The left half of the shield was designed to cover the unprotected right side of the hoplite next to them. The worst, or newest, fighters would be placed in the middle front of the formation to provide both physical and psychological security.
Roman legions used a type of shield wall called a testudo formation in which the first row formed a dense vertical shield wall and the back rows held shields over their heads, thus forming a tortoise-like defense, impenetrable to missile weapons. Although highly effective against missiles, this formation was vulnerable to being isolated and surrounded by swarms of enemy soldiers. Caesar, in De Bello Gallico, describes the Germans as fighting in a tight phalanx-like formation with long spears jutting oujt over their shields. It is this form of Germanic fighting that probably led to the shield walls of later armies.
In a phalanx, the man at the right hand of each warrior had an important role; he covered the right side of the warrior next to him with his shield. This made it so that all the shields overlap each other and thus formed a solid battle line. The second row was to kill the soldiers of the first line of an enemy shield wall, and so trying to break it. All the other rows were weight for the pushing match that always occurs when both sides tried to break the other wall. When a wall was broken the battle turned into a single-combat melee in which the side whose wall collapsed had a serious disadvantage.
In the late Roman army and the Byzantine army, similar formations of locked shields and projecting spears were called fulcum (φοῦλκον, phoulkon in Greek), and were first described in the late 6th-century Strategikon. Roman legions were typically well-trained, and often used short stabbing-swords (such as the Gladius) in the close-quarters combat that inevitably resulted when their shield-walls contacted the enemy. Auxiliaries were often less armed, therefore to provide more defence a shield-wall with spearmen was commonly used.