The People on the Precipice by Ian Watson.
The story was featured in the Evil Water: And Other Stories collection. Parts of the text are available in the Google eBook version.
The vertical wall is described as follows:
"Just suppose," he said, as the daylight dimmed, "that a whole world is as flat as a leaf! And suppose that creatures live within that leaf, who themselves are perfectly flat. Imagine this narrow ledge here simply carries on" - he chopped his hand into empty space - "in that direction forever! Imagine that it is a simple, infinite surface with nothing above it and nothing below it. And with no precipice to jut out from."
And later, a character's suggestion to climb upwards seems to be a big deal, compared to what's usually done:
"We make daily forays up and down for food. When we've scalped a patch we migrate sideways. That's life."
"And you'll die clinging on. Or rather, you'll die pretty soon after you stop clinging on. Now, today I'm climbing up to the Chief-of-Chiefs for that conference. Bounce will guard our ledge and keep the kids tied up. Loosepiton" - that's me - "will escort me upwards."