Answer:
Sumer was responsible for many inventions and important contributions to inventions of both the time and today. However, Sumerian city-states often fought with each other. They went to war for glory and more territory. This power - hungry people build walls to protect from invading armies looking to loot, pillage, and grow wealthy. The walls surrounded each city - state, closing them off from the rest of Mesopotamia. This lead to isolation, and more war. The city - states were constantly in a state of unrest.
Hope this helps! Have a great day! :D