I read a Sci-Fi novel a few years ago, it concerns a world with early iron-age technology that is a giant disc supported on an immense pillar. A cyborg is sent to study the people and report back to someone about them. They called the disc "the drum" as it made a certain sound when horses ran on the ground. They had primitive guns and rode horses instead of having vehicles. It was an artificial world, but it is not said who created it. There are mountains ringing the edge and one of the people accompanies the humanoid Cyborg to the edge to see it. Then he leaves. There were no sequels, it was just one book.