8 And now Limhi had discovered them from the tower, even all their preparations for war did he discover; therefore he gathered his people together, and laid wait for them in the fields and in the forests.
Limhi vigilantly watches from the tower and so sees the Lamanite armies assemble and is able to prepare for them. 
This contrasts greatly with the conduct of King Noah in the previous chapter. He flees up the tower to escape Gideon and only then happens to see the Lamanites  already within the borders of the land. His people neglected to watch the Lamnites and so missed the signs of marshaling of forces. They were too focused on the pleasures of the day and so failed to see the enemy approaching.
Limhi was vigilant and his vigilance allowed his people to fight back and to escape total destruction.

We too need to be vigilant. And we need to hearten to those who are on the watchtower and heed their warning and counsel.