Why was Mormon's Structure built?
Lehi and his children probably landed in America within a few years of 590 BC. Soon after arriving, Nephi (the Nephites) fled for his life from his brothers (the Lamanites) out of the land of Lehi (where they first landed and settled). Thus the land of Nephi was established. In the days of the first King Mosiah, the Nephites again had to flee north from the Lamanites. As they fled they discovered a city named Zarahemla that was built and inhabited by the Mulekites. The Mulekites had left Jerusalem at about the same time that Lehi had. The Mulekites became Nephites, and they lived together as one people (Omni 1:12-19).
By the end of the time of Captain Moroni (about 60 BC) the Nephites were in all of the lands North of Manti, and the Lamanites possessed all the land south of Manti.
Christ came to these people about a year after His resurrection. Until about 250 AD both Nephites and Lamanites went and lived wherever they wanted.
The prophet/general Mormon (for whom the Book of Mormon was named) was born 311 AD.
Land of Lehi
Land of Nephi
Mormon describes this land at this time as completely covered with buildings (Mormon 1:7). This probably includes the Darien National Park (Panama) area, now dense jungle.
When he was 11 his family moved from the lands north into Zarahemla. When he was 15 he was made general over the Nephite armies. They soon began retreating, eventually making a stand in Shem (Mormon 2:20,21). Mormon won the ensuing battle (349 AD), which was followed by 10 years of peace (Mormon 3:1).
In the peace treaty, the Nephites were given all of the land north of the narrow passage (where Mormon's structure is), and the Lamanites were given all of the lands south of this.
Mormon placed all his people's defenses at the narrow passage to keep the Lamanites completely out of the Nephite lands (Mormon 3:5,6).
This is when, where, and why this defensive barrier was built.
(Click here to see a timeline of events)