It cannot refer to the battlefield, since that lay in a valley. It could refer to the hill country where Zebulun and Naphtali lived, in which case the verse would be speaking of the self-abandon with which they left their territories for the battle.
But more likely the phrase refers to the slopes of Mount Tabor from which the tribes went to attack the enemy, even at the risk of their own lives.1
18 Zebulun is a people who risked their lives to the death; Naphtali, too, on the heights of the field.