While the experience of attacking a fortified city is apparently new to the Lamanites, they begin to learn what all other armies had learned, and that was that the attack would be long process. The entrance was sufficiently well defended that attack through that point was hopeless. What then became the concentration was to remove the dirt walls so that there was a larger breach in the defenses where the greater numbers of the Lamanite army could be brought to bear.
This attack on the walls was also one of the methods of defeat of a fortified city, but it comes only at the high cost of life, as the Lamanites found. Just as the attack on then entrance is repulsed, so too is the attack on the walls. The strategic advantage of having a protective barrier in front of the defenders, and the comparative advantage of gravity which would increase the force of the defenders missiles and weaken the force of those of the attackers which were launched upwards all had the effect of continuing the slaughter of the Lamanite army.