A shieldwall is a formation line of warriors standing shoulder-to-shoulder with their shields locked together, that is, with edges overlapping and held tightly against each other. This literally locks the line into place, and is extremely difficult to force apart. With sufficient skill and training, however, the shieldwall can be quickly opened and closed to allow other fighters to slip through the ranks, either to charge or retreat.
Fighters in a shieldwall cannot be attacked on their flank as they are protected by the shields of their comrades. From behind the shieldwall, fighters with polearms or bows can kill attackers in relative safety. When attacking a shieldwall, the goal is to punch through and disrupt the entire formation. This is most easily done by employing a tactic called a column charge.
The ends of a shieldwall are called the flanks and must be protected.