Phalanx formation was an ancient Greek tactic that involved soldiers forming a tight, dense formation to defend against enemy attack. There were many synonyms for this formation, such as shield wall, testudo, and tortoise formation. The shield wall was used by Norse warriors and involved soldiers standing side by side with their shields overlapping to provide maximum protection. The testudo formation, used by the Roman army, involved soldiers interlocking their shields above their heads to create a roof-like structure. The tortoise formation was also used by the Romans and involved soldiers forming a shell-like formation with their shields to protect against incoming arrows and other projectiles.