Arthur C. Clarke is known not only as a science fiction writer but as a hard science fiction writer, so when you begin to read "The Wall of Darkness" It can feel a little off not because of anything he writes but because of the unexpectedness of the story, but this is only fantasy in a small way. What this story really is, is a description of something we can see easily in two dimensions but is difficult to imagine in three.
The world take place in a world where travel between countries is slow and difficult. There are great areas that are impassible, and lands to the north which are barren, and then there is the wall. A great black wall that sits in the middle of the northern barrenness. The question of what is beyond that wall is one of the great philosophical question of the world. There are those that believe you will go beyond the wall when you die, and others that believe that we come from there. Those who are not religious believe that the wall was built by the engineers of an earlier age who understood the world better than them. Perhaps to keep out some dangerous enemy.
The story really begins when two young friends travel together to the wall. It is hundreds or possibly thousands of feet high, and black. It absorbs sound and there is no way past it, or so it seems. When the two return one of the boys discovers that his entire family was killed in an accident that destroyed their family home, and he has become one of the richest men in the world.
He lives a reasonably happy life, after the loss of his family, but the question of the wall eats at him constantly and he finally writes to his old friend who has become a master architect. Together with his money and his friend's skill they decide to scale the wall and eventually he does crossing the wall to find the last thing he had ever expected.
This story is a fascinating image of a world that has different rules than ours and while even after reading the story I can't fully imagine how it would work, neither can the characters in the story, which is why the wall exists. If you like stories that expand your view of not only our universe but how our universe could be then read "The Wall of Darkness".