Apparently Captain Moroni was constantly haunted by the prospect of a huge Lamanite invasion of the land of Zarahemla by a horde of Lamanites and apostate Nephites. It is apparent that his experience with the horrors of wars was driving him to undertake the most comprehensive fortification program in the history of the Nephites.
A representation of the types of fortifications which Moroni was building around every Nephite city may be seen on page 68 of volume three of Treasures from the Book of Mormon. Note particularly the manner in which towers were structured from which stones could be cast down on any who attempted to attack these strongholds.