I just had a very odd situation in a game against the ai that made me consider mountain terrain...
No sane commander would in the 18th century have marched a complete column with artillery and baggage across mountains, avoiding passes and roads...
So, would it be possible to deny access to mountains that are neither pass or road to wheeled units (artillery and supply)? That would still allow small forces, especially raiders to go through such rough territory, but not complete armies and major columnsand forces...