The earlier lightning attack on Ammonihah succeeded, at least in part, because the people of Ammonihah did not know that the Lamanites were coming. Ammonihah is on the western part of the land of Zarahemla, and apparently northwest of Zarahemla. This suggests that there was a mountain range that shielded the approach of the Lamanite army, as well as a pass that led from one side of the mountains to Ammonihah. Even though Mormon could not have known what was in the Lamanite mind when he suggested that they attacked again at Ammonihah because “they supposed that it would again become an easy prey for them,” it was not an unreasonable guess.
The last thing that the Lamanites, and perhaps the Zoramites, had known of Ammonihah was that it had been destroyed, and was, as Mormon called it, the “Desolation of Nehors.” Mormon indicated at that time that Ammonihah was abandoned “for many years” (see Alma 16:11). That was in the eleventh year of the reign of the judges.
Now in the nineteenth year, it had been rebuilt. Since the Zoramite-informed Lamanites did not know it had been rebuilt, it was in their minds of recent construction. This new defensive position consisted at least of the dirt embankment on the inside of the trench from which the dirt was taken, making a more formidable barrier. No mention is made of a wooden wall, but that would have been a typical part of the defensive structure.