It appears that there was some type of mountainous area, or other type of wilderness, in which Moroni might conceal his army on the west of Mulek. When Teancum makes a visible march to the north, they could be seen to be a much smaller army than was perhaps expected. Therefore, Jacob had his army leave the city for what he expected to be a relatively easy battle.
With significantly fewer men guarding the fortified city, Moroni was able to take his force and capture the city. As for Teancum’s army, they had no intention of actually facing the pursuing Lamanite army.