I have a question.
The most recent Civ game that I've played is Civ IV, which prohibited a player from ever moving any units onto or across mountain squares at any time in a civilization's lifetime. I considered this a game-breaker and sold the game.
My question, simply, is whether Civ V has the same or any similar restrictions on movement.
(I might ask the same in the Civ VI forum.)
I'm afraid you will be disappointed. In Civ V, most units cannot move onto mountains, most of the time. One civ (Carthage) can move ground units onto a mountain under certain circumstances.
In Civ VI, units may not move onto mountains as they do in Civ III. As PiR wrote, the military engineer (somewhat similar to a worker, but not available until certain techs as researched) can build tunnels to allow units to pass *through* mountains. Their presence affects other characteristics in cities, but they remain impassable.
Given that people have been traversing mountains for centuries IRL, I always found this element of Civ IV/V/VI to be weird and non-historical.