After the initial disarray due to the caltrops, the orc soldiers reorganized themselves. Each of the soldiers had their own way of traversing the caltrops field. Some went near the ones who had AOE skills that provided immunity or resistance against movement restrictions. Some got behind mages who used their AOE spells to blast the ground to get rid of the caltrops. Very few individuals, such as the Cipher Flight experts, used their mana sense ability to accurately detect the caltrops and destroyed them while they advanced.
Of course, this still slowed their advances down, which was what the caltrops originally intended to do. In this way, the war tools were not wasted. The human army also never stopped sending their ranged attacks, maximizing the damage they inflicted while the orcs' advance was being hindered.