Alma 49:16 - 49:20
Alma 49: 16 And behold, Moroni had appointed Lehi to be chief captain over the men of that city; and it was that same Lehi who fought with the Lamanites in the valley on the east of the river Sidon.
The city of Noah had put under the command of Lehi by Moroni, the commander of the forces that decimated the Lamanite army four years before on the east side of the Sidon in a previous invasion.
Alma 49: 17 And now behold it came to pass, that when the Lamanites had found that Lehi commanded the city they were again disappointed, for they feared Lehi exceedingly; nevertheless their chief captains had sworn with an oath to attack the city; therefore, they brought up their armies.
The Lamanites were surprised by the fortifications of Noah and they were especially disgruntled to learn that Lehi was the commander of the garrison. They remember the last time they had faced Lehi and his fighters. Now they may have marched off to attacked another place, but their commanders had sworn an oath to attacked Noah regardless of it defenses so they brought up their men to prepare their attack.
Alma 49: 18 Now behold, the Lamanites could not get into their forts of security by any other way save by the entrance, because of the highness of the bank which had been thrown up, and the depth of the ditch which had been dug round about, save it were by the entrance.
The problem the Lamanites faced was there was no way into the fornications around Noah accept by heavily defended narrow entrance. The Nephites had built high walls of dirt which was dug out of the ground around the city which created a deep ditch. This wall surrounded the city except for the narrow entrance which was the only way into the city.
Alma 49: 19 And thus were the Nephites prepared to destroy all such as should attempt to climb up to enter the fort by any other way, by casting over stones and arrows at them.
So if anyone tried to climb up the dirt walls they could easily be picked off by the defenders with their arrows and slings.
Alma 49: 20 Thus they were prepared, yea, a body of their strongest men, with their swords and their slings, to smite down all who should attempt to come into their place of security by the place of entrance; and thus were they prepared to defend themselves against the Lamanites.
The entrance was also defended by the strongest men in Lehi’s forces. They were prepared to kill anyone who would try to enter the city through this narrow entrance. To sum up the defenses of the city of Noah consisted of a high wall of earth with a deep ditch in front of the walls and a narrow entrance defended by the strongest men of Lehi’s forces.