These chapters regarding war between the Nephites and Lamanites contain many lessons, but they also seem to me a testimony of the truthfulness of the Book of Mormon. It's hard to imagine such a complicated history of political intrigue, military strategy, and detailed military tactics being entirely invented by a poor farm boy with little formal education.
One lesson from chapter 49, is that we need to prepare ourselves for the attacks of our adversary. We can build fortifications in our lives to protect us against temptation. The narrow entrance to the Nephite fortifications makes me think of the need to restrict the access of media to our homes. If there is only a narrow entrance and we guard that entrance, we can protect our homes from the evil that Satan makes available through cable and the internet.
President Spencer W. Kimball spoke of fortifying our homes in April General Conference, 1979:
"We need continually to fortify our homes and families and defend them against the onslaught of evils such as divorce, broken families, brutality, and abuse, especially of wives and children. We need to constantly guard against immorality, pornography, and sexual permissiveness that would destroy the purity of the family members, young and old.
"Such evils are very real and very threatening. One has but to read the headlines of our newspapers and magazines to become frighteningly aware of the crumbling, destructive influences which surround us.
Perhaps I sound like an alarmist. If so, it is because I am alarmed. I am greatly concerned, and so are my Brethren in the First Presidency and the Council of the Twelve Apostles and others of the General Authorities.
"If we could but suggest you go home and lock these evils out by closing and bolting the windows and locking the doors of your homes securely, it would be a simple matter.
However, such security would be ineffective against the evils of which we speak. They come into our homes on ether waves by radio and the television screen. We find these evil forces almost everywhere we go. Exposure is almost constant. We track them into the home from the school, from the playground, from the theater, the office, and the marketplace. There are but few places we go in our everyday world where we can escape them.
"What then must be our service? What must we do? We must be constantly alert to their evil presence in our homes and destroy them as we would the germs and filth of disease. We must hunt them from the closets of our minds, freeing ourselves of such worldliness, quenching the embers of wickedness before they become destructive flames. How do we do this?
"If we would escape those deadly thrusts of the evil one and keep our homes and families free and solidly fortified against all destructive influences so rampant about us, we must have the help of the very founder and organizer of this family plan—the Creator himself. There is only one sure way and that is through the gospel of the Lord Jesus Christ and being obedient to its profound and inspired teachings. Surely we must be made to realize that the purchase price of a family hearth free of such evil influences is the keeping of the commandments of God."
Spencer W. Kimball, “Fortify Your Homes Against Evil,” Ensign, May 1979, 4
Therein lies our greatest protection -- keeping the commandments of God.