"This giant wall of boulders can be destroyed."—In-game description
Boulder walls are extremely tough walls, even against siege weapons, such as Petroboloses, as they combine enormous hit points with very large resistances.
Unlike the Wall of Atlantis, it is not completely invulnerable. Still, it usually takes much effort to demolish them, and even more if there are no siege weapons nearby.
As mentioned, they are featured in some scenarios, usually to deny passage to the player, but in one case, they are entirely decorative; in the scenario War of the Titans, four Boulder Walls clamped together represent the site from where Gaia will emerge to defeat her son, Kronos.
In the Tale of the Dragon expansion, there is an Asian version of the boulder wall, which is more brownish in appearance. It has identical hit points and armor.
This pile of rocks is not natural. The huge stones were moved by giants - or by men at tremendous effort - to block unwanted passage.