Mounted units may cause Flank Attack damage to all Siege Units in a stack. When attacking a stack with Siege Units, the Mounted Unit will inflict this Flank Attack damage so long as it survives combat, whether it retreats or wins. If it dies, no Flank Attack damage will be inflicted. So when you see the enemy approaching with strong siege weapon stacks, give your mounted units the Flanking promotion, so that they have a higher chance of withdrawal and thus a higher chance to inflict damage on the siege weapons.