Chief Captain Moroni understood that when Amalickiah escaped capture, he would both go to the Lamanites and he would likely stir them up to invade. Moroni could not have anticipated how dramatically Amalickiah would rise in power and ability to direct the Lamanite armies, but Moroni still thought it prudent to defend the land with more than just men. In addition to strengthening the armies, Moroni erected “small forts, or places of resort.”
The idea of the small fort was that it was a defensible position that served only a military purpose. These were not walled cities, but military outposts. When the text says that he “[threw] up banks of earth round about,” this was a typical means of simple fortification. A pit was dug around the area to be protected, and the dirt removed from the pit was built up on the interior side of the trench.
It is interesting that Mormon also mentions stone walls. While some locations would have had them, some might have had the more typical wooden walls erected on the top of the mounded dirt. Both types of fortifications are known archaeologically in Mesoamerica, though most of them are attested in later times.