Geography of ancient Mesopotamia
Mesopotamia is located in strange area for such a fertile land. It is surrounded by mountains in the north, and deserts to the south. The Zagros Mountains are a key mountain range in Mesopotamia. They helped to keep invaders out, as it was hard to bring the necessary materials for war over a mountain, and still have enough energy to fight.
The Syrian Desert (to the south) also helps to keep invaders out. Forces attempting to siege the area would exploit their resources and be unable to keep up the energy needed to launch a full assault.