When a building is being knocked down and replaced with another, the construction workers usually don't clear the rubble. They just flatten the rubble and build

When a building is being knocked down and replaced with another, the construction workers usually don't clear the rubble. They just flatten the rubble and build ontop of it. That means that cities like Rome and Vienna truly are building up, ontop of centuries of different buildings. If you look at walkways down to the Tiber river in Rome, you have to go down two or three flights of stairs to get to the river. I imagine that Rome was originally flush or close-to-flush with the river, and now it's 3 stories taller.