Shieldwall

The formation of a shield 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 soldier benefits from the protection of their neighbours' shields as well as their own.

This tactic was known to be used by many ancient armies including the Persian Sparabara, Greek hoplite, Macedonian phalanx, and Roman legion, though its origin and spread is unknown. It may have developed independently more than once.

Although little is recorded about their military tactics, the Stele of the Vultures depicts soldiers in a shield wall formation during the third millennium BC.

By the seventh century BC, shield walls in ancient Greece are well-documented. 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.

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's purpose was to kill the soldiers of the first line of an enemy shield wall, and thus break the line. All the other rows were weight for the pushing match that always occurred when each side tried to break the other's 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.

...
Wikipedia