Mormon’s comments about Amalickiah not caring for shedding the blood of his own people becomes relevant here as “their chief captains had sworn an oath to attack the city.” The city of Noah was not only perhaps better defended than Ammonihah had been, but it was also defended by a general, Lehi, who had been part of the army that had soundly defeated the army of Zerahemnah. Nevertheless, the Lamanite chief captains attacked.
We learn a little more of the nature of the Nephite defenses. In many ways, they were similar to defensive fortifications all over the world. Many principles are the same. Since the walls surrounded a city, the people needed a way in and out. In this case, it does not appear that there were gates. The entryway appears to have been open. However, the entrance would have to be somehow curved, or turned a corner, so that there was no way to have a direct assault on the opening.
The elongated entryway also allowed armed men to take up position on the tops of the walls on either side. Thus, in verse 20, there are strong men standing at the entrance. There were defenders along the walls (verse 19), and certainly men on the top of the walls of the path leading to the entrance, making that entrance a killing zone should any army attempt to attack at that location.