Haftvad's Ambush in the Mountain Fold

From D6.13, Ardeshir Fights Haftvad and Is Defeated. It illustrates D6.13.4–8:

D6.13.4
He hid his troops within a mountain fold,Then marched to battle with his warriors bold.
D6.13.5
When battle raged, and gorzes, face to face,And battle-axes thundered in that place,
D6.13.6
The hidden troops emerged to join the fray;For Iran’s chiefs, the earth grew black that day.
D6.13.7
No man could tell his hands from feet; the groundItself, it seemed, had held their feet fast bound.
D6.13.8
On dale and mountain such a slaughter grewThat even victors tired of those they slew.