3 Nephi 3:14

And he caused that fortifications should be built round about them, and the strength thereof should be exceeding great. And he caused that there should be armies, both of the Nephites and of the Lamanites, or of all them which were numbered among the Nephites, should be placed as guards round about, to watch them, and to guard them from the robbers, day and night;

He had them build very strong walls around them. He had armies from both the Nephites and Lamanites stand guard around them. All those who were counted as Nephites were guards. They watched day and night to keep the robbers away.

He ordered massive fortifications built around them, incredibly strong. He positioned armies—Nephites, Lamanites, everyone counted among the Nephites—as guards around the perimeter to watch for the robbers and protect them day and night.