The story
From ladder + wall from the construction of the wall akin to a ladder and the action needed to pass the obstacle, climbing it like a ladder.
Etymology from Wiktionary, CC BY-SA 4.0.
ladder wall
Today it means: A type of wall obstacle in obstacle course racing composed of a vertical wall of crossbeams with space in between the beams, where competitors must climb up the wall, go over, and climb back down. Full definition →