I'd like two medieval societies to be separated from one another by a huge vertical wall.
The wall is so high that no-one has succeeded in shooting an arrow (or anything else) over it. It disappears up into perpetual mist. This wall bridges a pass in the middle of a mountain range that no-one has managed to scale because of the continual blizzards that rage and indeed the traditional stories about fearsome beasts that live there.
This wall is solid. There is no magic involved - it was built by an ancient civilisation.
The present-day civilisations can see each other through the wall but can only communicate in sign language. In fact the only people really interested are the children who spent a lot of time making faces and throwing stones at each other, all to no effect. Also of course there are medieval tourists who come to gape at this marvel.
The problem is that I want it to be smooth, unbreakable, unclimbable but most of all transparent.
What can this wall be made of? Could it be glass, diamond, some other real transparent material? How thick would it be?
A huge wall of ice would not be transparent, and would constantly need to be replaced at the top because of melting at the base. So I exclude that unless someone can prove me wrong.
To sum up: The wall must have survived for thousands of years, still be transparent (at least if given a wipe) and strong enough not to have been substantially damaged.
Without the use of magic and using science that could really be done by an advanced civilisation with enough resources, can the wall be self-supporting and what should it be made of?
EDIT - I should have provided some idea of the shape and scale of the mountain pass. I won't change anything here because I don't want to invalidate any answers. However if you look at my own answer (vertical conveyor belt) to my question below, I've added some links to pictures giving an idea of what I was imagining.