Moving forward

There was this fantasy book I read once, must have been when I was a teenager. I can't remember what the book was about, who the protagonist was, or the title. All I remember is the description of one city that the protagonist encountered.

It was a massive moving city. The city was carried on the backs of the countless people who lived underneath it, who were whipped when they didn't move forward. When people under the city died, which was inevitable due to the conditions, they were replaced by people from the city who had fallen too far into debt. The people under the city, their only order was to move, there was no pilot, no steering wheel, no track, they were just moving forward.