Military history is full on encounters where one side uses terrain the other side believe to be impassable.
First rule of combat.
Impassable terrain is never impassable unless you defend it.
Baghdaddy couldn’t be more right here. :-)
Another famous example that comes to mind occurred on Bataan where the extremely incompetent Gen. MacArthur failed to defend the “impassible Mount Natib”, therefore losing his stronghold on Luzon.
From Wikipedia:
“Mount Natib, a 4,222-foot-high mountain that split the peninsula, served as the boundary line between the two corps. The commanders anchored their lines on the mountain, but, since they considered the rugged terrain impassable, they did not extend their forces far up its slopes. The two corps were therefore not in direct contact with each other, leaving a serious gap in the defense line.”
http://en.wikipedia.org/wiki/Battle_of_Bataan