In the age of medieval European castles, sturdy walls were indispensable. The surviving ruins of ancient European castles were mostly built of stone; earthen ramparts were rarely seen.
Earthen walls, however, were common in Asia, especially in regions lacking stone. Unless one spared no expense as the egyptians had when building the pyramids, transporting stone from afar was simply too costly.
It certainly consumed manpower, materials, and money, but without walls, city dwellers had no sense of security. At the time, only important cities such as Jerusalem and Constantinople built walls entirely of stone.
Paris's prosperity continually attracted new threats, such as the Vikings and eastern heathens. During the reign of King Philippe Auguste, France needed to expand its forces to defend against the Arabs while also reinforcing its defenses. Building everything from stone was impractical, so people came up with a compromise: rubble and bricks were mixed into the walls, then covered with a layer of stone on the outside. No one knew whether they were truly sturdy, but at least the walls looked solid. At the time, the Louvre Museum was not a palace for displaying art, but part of the city's defensive system. There were also pastures, farmland, and vegetable gardens to ensure the city could maintain its food supply during a siege.
If mud was too watery, it could not hold the bricks when plastered onto a wall. If the bricks were improperly fired, they likewise could not be used to build sturdy walls.
Napoleon himself had originally been Italian, yet after his abdication, while imprisoned on Elba, he had once told British visitors that people in Italy were lazy and weak, while the people of France, at least in Napoleon's time, were different.
Anyone could go to the front lines, except Victor Hugo. He had