When Kaleb and Arthur noticed the large arrows of mana coming from the beginning and end of this corridor, they both realized the mistake they had made.
By circling their defenses to protect themselves from the offensive formations in that corridor, they were confident they could withstand the offensive power of Vicente's trap. Offensive formations in places like this were not designed for continuous attacks. They aimed to kill or seriously injure as many opponents as possible in a quick attack.
For this reason, simple direct attack formations like the one in that corridor only worked for a few moments before they had to be reloaded. That could take several minutes or even hours, so after the initial attack, they would temporarily go into standby mode.
One could be sure of one's life or purpose by defending against this type of formation. The chances of anything else happening would be minimal, which gave Kaleb and Arthur confidence in their actions.
