In modern times, it seems to be standard practice to demolish/remove existing structures before building on top of a choice plot of land. But how did some of the older structures throughout time end up being buried under layers and layers of civilization?

I understand that some of it is environmental - a city that was partially abandoned may have had accumulations of dirt piled on top. But how about a city like London or Jerusalem? Were new structures just built on existing ones, combined with roads being raised to meet the frontages of the new buildings?

Part of what prompted this question was the discovery of the Greek citadel recently in Jerusalem. Just how did that manage to get buried under modern progress?