This is a great tweet and link:

The link is to a figure-ground map of Paris that allows you to filter its buildings by period of construction. Here's what all of the periods and all of the buildings look like:

Once you play around with the map, it will become obvious that the second half of the 19th century and the early 20th century was a prolific building period for Paris (1231 hectares of area). This is what Samuel was getting at in his tweet.

I would love to see a map like this for every city in the world.