Fortifications: Why walls of stone and not of earth?

[Robert Vermaat]

Yes, but the amazing thing is that mud brick and rammed earth walls are on the other side pretty invulnerable against otherwise effective high tech assault tactics, namely sapping and catapult shelling! More, much more than stone and fired bricks.

Goes to show the action and reaction of defences and siege science. Stone makes better walls, leading to the development of better siege techniques. When cannons were invented, it was back to 'good old' earth again!