Tighe lives on the wall. It towers above his village and falls away below it. It is vast and unforgiving and it is everything he knows. Life is hard on the wall, little more than a clinging on for dear life—and then one day Tighe falls off the wall. "On "is at once a vertiginous concept novel, a coming of age saga, a picaresque journey across a changed world, and an epic adventure in the very best traditions of science fiction.