Gandalf has appeared leading Eomer and the Rohirrim to save the King and his company. See how Gandalf is an embodiment of light (see below). Gandalf is himself the white banner of the kingdom of Rohan. The enemy forces appear as a coalesced black mass enhancing the contrast between good and evil. Evil is already shrinking back.
Falling to Rise
The slope before Gandalf and the Rohirrim appears to be impossibly steep. Somehow the riders come on at an impossible gallop over terrain so acutely steep they might be expected to fall. Yet Gandalf is leading them. We are reminded by this precipice of Gandalf’s fall into the depths of Moria and the profound and mystical battle that took place there. Gandalf fell in Moria yet this precipitated a rise to greater power. The ‘falling so he will rise’ motif is recalled her in the charge to Helm’s Deep. There is also a hint of Aragorn’s fall on the way to the Deep. The radical and outwardly impossible angle of the ride of Gandalf here recalls past victories.