At the intersection of rue Eugene Sue and rue Simart in Paris is a collection of mid-rise buildings that were constructed during Haussmann's renovation of Paris in the second half of the 19th century. They were intended to house some 10,000 workers.

Here's an aerial image of that intersection from Google Maps:

Some people, namely the folks over at The Funambulist, have compared this area to the Kowloon Walled City in Hong Kong as a result of its extreme population density. But unlike the Walled City, this area of Paris still exists today.

The way these blocks are able to achieve such a high population density is by employing compact internal courtyards. In some cases they would be more accurately referred to as light wells.

Here's a fascinating diagram from The Funambulist showing the approximate areas of each courtyard/light well:

I have become very interested in these sorts of European courtyard buildings. They do have their benefits. For one, they offer respite from the rest of the city. But you can also understand why the modernists were so fixated on access to light and air.