It occurred to me the other day that creating a flight of stairs or a ladder is not a simple invention. At what point did humans go from having single story buildings (houses or otherwise) to having buildings be two or more stories? What did they use to get to the top?
Very early.
The cities Harappa and Mohenjo-Daro in the Indus Valley Civilization (which existed fro 3000 BC-1700 BC) which can now be found in modern-day Pakistan and northwest India were excavated to have several multi-storey buildings.
And an interesting feature of the very early city of Çatalhöyük in modern Turkey which existed from 7500 BC to 5700 BC was that ladders from the ceiling were used to enter the houses instead of doorways.