This is a tad bit too unrealistic.
In history, mountains were not absolutely impassable. Hannibal's overland march to Italy, anyone?
In history, mountains were not absolutely impassable. Hannibal's overland march to Italy, anyone?
This really has to be one the most overused example in civ history and I bloody well know why: there is no other decent one!
Can't model everything that happened just once in history.
Anyway, gameplay should and does triumph reality, so...

But, I like the system the way it is now. MAYBE there should be a special unit that can "hoof-it" through the mountain,
Don't disagree with you here but!!
What if the AI enters my borders with such a unit and places it on a mountain and we're at war, how am I going to get rid of it?
If only one sort of unit gets to enter mountains, it means only the same type of unit could kill it, now that's a bit of a problem (and annoying cause I may have to build several of those units that are perhaps useless or outdated), isn't it.
In game the explorer can only defend (IIRC) so that's not going to even work.

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