‘ And they set the people, all the host that was on the north of the city, and their liers in wait (literally ‘their heel') who were on the west of the city. And Joshua moved that night into the midst of the valley.'
This probably refers to the officers putting the main army in order ready for battle, and the preparing of the group of five eleph. It is clear that we are to see the thirty eleph as remaining in secret and therefore not at this stage involved in operations. Then the main army moved down into the valley during the night. Their ‘heel', the five eleph, were set by their officers for whatever was to be their part in the battle. ‘Heel' may indicate a rearguard, or those who would provide reinforcements when needed.