Both sides have prisoners and apparently Moroni, at least, believes that an exchange is possible.
Geography: The land Bountiful is immediately south of the narrow pass. By fortifying the land of Bountiful, Teancum would be protecting the narrow pass as well. This goal is important to Moroni, since he does not want the Lamanites able to “harass them on every side.” If the Lamanites held the narrow pass, it would cut off a possible Nephite retreat to the north. The Lamanites are already in a very strong position, but are not yet able to “harass… on every side.”