The invasion route leads through a narrow mountain pass. There is no way around the enemy now.
The Hero crossed the river quickly and has gained the high ground over Chaos, before its forces can properly establish themselves in the pass.
The pass is inclined, allowing them to add a d6 to charge distances and a +1 to CR to those units with their backs towards the Hero's side of the battlefield, but must fully deploy first.
This quest's result: