In Alma 50, Moroni was attempting to fortify the land of Zarahemla against the Lamanites during a time of internal instability brought on by the king-men. One of these king-men named Amalickiah had previously fled to the land of Nephi and by treachery had become king over all the Lamanites. As king, his goal was to eventually "overpower the Nephites, and to bring them into bondage" (Alma 48:4). Both Amalickiah and Moroni saw that the eastern and western coastal areas surrounding the land of Zarahemla were critically important. Mormon writes:
Thus Moroni did prepare strongholds against the coming of their enemies, round about every city in all the land.
And it came to pass that Moroni caused that his armies should go forth into the east wilderness; yea, and they went forth and drove all the Lamanites who were in the east wilderness into their owns lands, which were south of the land of Zarahemla
And the land of Nephi did run in a straight course from the east sea to the west.
And it came to pass that when Moroni had driven all the Lamanites out of the east wilderness, which was north of the lands of their own possessions, he caused that the inhabitants who were in the land of Zarahemla and in the land round about should go forth into the east wilderness, even to the borders by the seashore, and possess the land.
And he also placed armies on the south, in the borders of their possessions, and caused them to erect fortifications that they might secure their armies and their people from the hands of their enemies.
And thus he cut off all the strongholds of the Lamanites in the east wilderness, yea, and also on the west, fortifying the line between the Nephites and the Lamanites, between the land of Zarahemla and the land of Nephi, from the west sea, running by the head of the river Sidon the Nephites possessing all the land northward, yea, even all the land which was northward of the land Bountiful, according to their pleasure. (Alma 50:6-11, italics added)
In these verses, Mormon informs us that the Lamanites had "strongholds" in the east wilderness, an area Mormon had already described in Alma 22:29 as being the wilderness "on the east (of the land of Zarahemla) and being inhabited by "the more idle part" of the Lamanites. [Alan C. Miner, Personal Notes]