As the construction of the wall finished, and the second phase of Alexander's siege began, he transferred all his soldiers and artillery onto the wall to give them a better advantage.
This helped the crossbowmen and the ones manning the scorpions as now their shots did not need to fight gravity to reach their target.
Meaning many of the bolts that previously would have lost all kinetic energy by the time they had reached their target and simply fell from the sky, pinging off the defender's armor could now have enough force in them to penetrate and wound or kill them.
Along with these, Alexander also employed his catapults, which he actually saw to be the main star of the show, for he had invented quite the nefarious ammo to be used with this missile delivery mechanism.