Macedonian phalanx
The Macedonian phalanx was impregnable from the front, but vulnerable from the sides and back, where it could be attacked by the enemy cavalry. Therefore, the system required protection, which it got from the Macedonian cavalry, which was the most flexible force. At the start of a battle, Alexander used the phalanx to pin down the enemy. His heavy cavalry would drive the enemy horse from the field, and then charge selected opponents or exposed enemy unit flanks. Then, the phalanx would move against the centre.