You'll probably see the same scaling mechanics from Skyrim and Oblivion. There is the ability to scale fortress walls in Cyrodiil, whether or not that is exclusive to that particular event (sieges) is anyone's guess.
They have not announced exactly how the zones will be made up. But I would suspect that you'd be able to climb anything within reason. If it a mountain that is following the zone wall (meaning nothing is over the other side) you'd be unable to too. But if it was a mountain that is dividing zones I would suspect you'd be able to it may require a bit of time and effort to do so since it is a mountain after all there won't be a path
Has it been announced that zones have a loading screen in between them?