Why is there a wall surrounding Uruk in the novel, Gilgamesh: Man’s First Story?

Uruk is surrounded on all sides by high walls meant to protect it and its people from beasts, natural disasters, encroaching armies, and unfriendly gods. The greatest danger to Uruk, however, and the primary reason for the walls, is the monster Humbaba who rains down fire and ash constantly from his nearby mountaintop.