What geographic feature of Mesopotamia lead to the Sumerians being easily taken over?

The correct answer to this open question is the following.

The geographic feature of Mesopotamia that led to the Sumerians being easily taken over was the Tigris and Euphrates Rivers that made it easy for their enemies to navigate and invade them.

Sumeria was the oldest civilization known to man. Sumerians settled in the middle of the Tigris and Euphrates Rivers in the Middle East region. There, they developed agriculture techniques and learn how to use the flood of the rivers to grow crops in that fertile soil. That is how they prosper for some time until their enemies could access their city-states navigating through both rivers.

arresting and exicuting citizens

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.