It is again worth noting how much time Moroni had to prepare his defense before the Lamanites arrived. He called up more men, and then organized them into two groups and placed them in their desired locations. What Moroni does is split the army, concealing one part on the east, where they could be hidden by the hill Riplah from an army coming down from the north.
The other army is hiding in the west valley. The intent is to bring the Lamanite army between the two halves of Moroni’s forces, one on each side of the River Sidon. Crucial to this story is that there must have been a known ford where even the Lamanites understood they could cross the river on foot.