Alternatively, you could give Rangers the ability to create a new "mountain pass" improvement that would allow one unit at a time to pass (maybe exclude siege weapons, chariots, etc.). This would only be buildable when there is passable terrain on either side, so you couldn't have a pass through the center of a huge range. Give it a small penalty on defense but a huge bonus to fortification (so your troops are vulnerable when passing through, but you can hold it if you are prepared).