I've little experience with this, but if there's a certain height that you wouldn't want the player to pass on the mountain or hillside somewhere, then you could create an impassable invisible map or model entity(like a box or rectangular prism) around the ridges you wouldn't want the player to pass. You could also directly code it by detecting the slope of the hillside/mountain and restricting movement past a certain slope value but I wouldn't know how to determine the slope of the surface in 3dgs; I just know that, at least in the A5 template script, there was coding that detected the underlying slope value(or, rather, the perpendicular form of that, the normal vector) and that was used to determine force values for the player(to slow down the player while climbing up a hill, increase speed when falling down, etc.). So it's possible!
