Just as the Lamanites did not know that Moroni had rebuilt Ammonihah (at least as a “fort of security,” if not a thriving city), they did not know that Moroni had significantly reinforced the defenses of the city of Noah, which had previously been a weakly defended location.
The Lamanites had hoped to find an easy way into the heart of the land of Zarahemla, but Chief Captain Moroni had reinforced many cities to prevent just such an easy invasion. Therefore, the Lamanites were faced with yet another fortified city, and probably understood that this is what they would continue to find.