Imagine living in a city with no sewer system. A lot of waste is disposed on the streets regularly. I am thinking oof examples in European Antiquity here. So, in addition to chamber pots and kitchen rubbish, there would be various animal droppings. If roads are not paved, when it's the rainy season, streets become very muddy. So some more earth or gravel is put on the streets to make them less muddy. Because of This, the street level slowly rises up. So maybe what once was your ground level entrance now lies one step under the streetlevel.

When looking at the history of citis, you often find recordings of fires throughout parts of the city. Moreover, there might be destruction through earthquakes, flooding or war. When you have a lot of destroyed buildings, it is often easier to flatten the rubble out and build your new homes on top. Sometimes, the structures on the ground floor survive, but as the accumulations in the streets rose, what was once a ground floor now becomes a cellar.

Building activities without technical or mechanical help are much more difficult,