Most cities stay put — they're parts of the landscape like mountains, rivers and forests, and one expects them to stay in the place where they were built. Most fictional cities follow this same assumption, but some — chiefly those in more speculative fantasy and science fiction settings — buck the trend and move around. The least fantastic version of this is an exaggerated version of how nomadic societies work in real life. A large group of people moves around, carrying tents, large carts.