Moroni appears to understand that the attacks from the Lamanites are not over. We do not know how he understood this, though certainly inspiration is a probable source. It is also possible that he was an observer of the times. This time period is where the foundations of the Classic are being laid, and the rate of militarism appears to have been increasing in the Maya world. Moroni may have been able to see the trends in surrounding populations and read those increases in militarism as a long-term trend rather than an aberration. Certainly we have greater and more frequent large-scale attacks by the Lamanites at this point in history than at any time previous.
Because of Moroni’s understanding of the probability of continued attacks, he continues to develop the fortifications that were so effective in this most recent rejection of the Lamanites. The ridges of earth around the cities are a continuation of the defensive policy we say at Ammonihah and Noah. It is not clear whether the placement of the timbers at the top of the mounds was also employed at Ammonihah or Noah, or whether this is another innovation for this round of construction. In any case, as we have seen, this type of defensive system is know for the Mesoamerican region. While Moroni was an innovator, his innovations would have been widely copied once their value was learned. We have already seen this process of copying in the armor employed by the Lamanite army that attacked Noah (Alma 49:6). We may assume a similar diffusion of this defensive scheme throughout a larger area.
Textual: A question that may be asked at this point is why Mormon chose to create a new chapter with this information. We have seen him finish the story of his major character in a chapter before, but this time the story of Moroni extends across a chapter border. Why?
In this case, we have a few changes that allow Mormon to shift chapters. The first is the year break that gives him an opportunity to finish one story and move to the next. This is a weak reason, however, because he has had multiple years in the same story before. What is more likely is that this chapter begins a different phase of the wars with the Lamanites. The last chapter dealt with the military invasion of the Lamanites, and ended with the righteousness of the people. In this chapter we will see different types of problems, which begin internally. Thus we have a shift from a righteous people to a people with a large number representing unrighteousness. That shift in the relationship of the people to God would give Mormon reason to change chapters.