Moroni had a year to continue his campaign of strengthening the Nephite defenses. Having taken possession of the eastern seaboard, he fortified the coast to protect the Nephite lands from any army that might attempt to come up the coast to enter deeper into Nephite lands.
Although we typically hear of threats from the Lamanites on the south, there must have been at least some possibility of a threat from the north, because Moroni fortifies the north as well. That fact continues to emphasize that there were more people in the land than those directly mentioned in the Book of Mormon. These people in the north will become more important in the book of Helaman.
In verse 15, we have a reminder that the punctuation of the sentences in the Book of Mormon was not part of the inspired translation. The words were, but the compositor at Grandin Press created the punctuation. In this case, it makes it appear that there was a type of city that they called Lehi. It is more likely that there was a certain type of city built, and that city was named Lehi. Thus, it would be better to add a comma: “one in a particular manner, which they called Lehi.”