Shield Walls

The use of shield walls during a battle dates back to the ancient Greeks, whose Hoplites used this as part of a phalanx. They were able to use spears & swords from behind the shields while protecting themselves & other members of their units. The Roman legions were trained in similar tactics using their shields (sutum), including their famous testudo manoeuvre that also covered the sides and tops of their units.
These have since been adopted by the Germanic peoples, especially the Saxons, Anglo-Saxons, and Norse making good use of this tactic. The only fault is that if penetrated the formation cannot be reformed, and the unit can break & flee when this happens. A common approach is to use fire based attacks, or mounted attacks against them, preferably from the sides or rear. Infantry primarily use this tactic, and the Franks have developed the Francisca throwing axe in response to it. Sometimes elite units use it, where they are more dependable, and harder to break, as well as many training to build up morale & the ability to shrug off all but the worst of the attacks.