Archeological ruins are often underground. How do cities get built on top of other cities?

Answer 1

Continuous occupation of the same site or reoccupation of a site that is situated in an attractive setting.

It is usually a progressive thing, not cleanly defined layers unless an entire city was laid waste by some catastrophe.

Answer 2

Older cities were often built of mud brick and clay. If the city or part of the city is abandoned (earthquake, flooding, fire, bad luck, invaders, sickness), then the upper portions of the buildings collapse into the lower portions. Except in extraordinary situations, like Pompeii, whole buildings are not found, just the lower walls and floors of rooms.
Also, people may intentionally knock down buildings and instead of removing the building, build on top of it. I just read a book on the center of Rome which discussed how we know about earlier buildings because the maker of a later building knocked down the walls of the older then brought in dirt to make a level platform ("hill") for the new one.
Also, if you have never visited an older town, most of the old places were built for people walking or using mules and thus have very little space between the buildings. Assorted trash, manure, etc. end up in the street, so the actual street level rises until finally, someone decides that this old house has been around long enough, they knock down the walls and build a new one above the raised street.

Answer 3

We are doing it every day. Every time we tear down a building and put a new one in its place, or a building burns down and we build on the same lot, we are leaving material for future archeologists.

Answer 4

Location, location, location. for some reason that spot has appeal for a settlement and the old ones just get coverd by dirt and erosion deposits.