A wedge can break a tortoise provided the wedge has at least 2 men for ever 3 in the tortoise. Otherwise, the shield wall will wrap around the sides of the wedge or phalanx. A phalanxe or wedge must have 2 men for every 1 in a shield ring in order to break the formation. A shield wall can attack a phalanx, wedge or shield ring and envelop it. A shield wall will stop a tortoise and force it to change formation on the next turn.
Shield walls can be square, composed of many ranks behind the first.
Only the first three ranks are counted when formations clash. Scattered troops fight at minus 1 against a formation. Obviously, you want to keep your units together. You want to maintain some kind of formation. The goal is to keep your forces organized and in formation while smashing the enemy formations.