Joseph Smith lived in a place where new settlements had sprung up in the wilderness, and he was quite familiar with what it took. Americans of his time and place new perfectly well the difference between a settlement, a village, a town, and a city. They knew from hard experience exactly what it took to get from wilderness to city -- how many years, how many settlers.
So why is it that when Captain Moroni is trying to secure the Nephite borders, he founds cities left and right? Cities that are instantly fortified and populated, seemingly without any kind of migration of settlers from the heartland of the Nephites? Joseph Smith knew the word fort, and new perfectly well that a military commander trying to secure hostile country would not found cities -- he would build forts and leave garrisons in them.
But in Meso-American culture, particularly in the jungles where the Mayans built their cities, there were always small villages and settlements in the wilderness, families on their own, and an ambitious king looking to expand could very easily move into a new territory, gather together a lot of isolated and vulnerable settlers, and unite them into a city. No settlers were needed from outside. The people were already there. They simply gathered together to create public works -- temples, in Mayan times, but in Moroni's case, fortifications for defense. The cities didn't require large garrisons, since the newly gathered people became citizen soldiers. And many people would gladly accept the overlordship of the people who gathered them and made a city out of them. It gave them greater safety, a sense of identity, and bonds with a great nation and desirable culture, not to mention a powerful religion.
Nobody in 1820s America knew anything about this. Yet Captain Moroni goes through the wilderness building cities exactly as Mayans could and did. And the author of the Book of Mormon didn't even make a big deal about it -- just took it for granted as being the way the world worked. Of course you can build instant cities in the wilderness, without bringing in outside settlers. So it seemed to Mormon. But could it possibly have seemed that way to Joseph Smith?
From "Artifact or Artifice" by Orson Scott Card.