The ability to work with stone and use it as a construction material marks another important milestone for an early civilization. Stone buildings are infinitely more stable and resistant than wooden or clay ones, and offer unparalleled opportunities to defend a settlement from foes. Of course, this development also prompts the development of the means to break down these new defenses, manifested by the Battering Ram. At the same time, the uses of a whole new type of minerals are revealed, and your Builders may now construct Quarries to access them.