The geographical feature that served as the boundary for the ancient Roman Empire was the Rhine and Danube Rivers in the north. These rivers formed natural barriers against incursions from the tribes in Germanic regions, helping to demarcate the limits of Roman expansion. The empire extended across Europe, North Africa, and parts of the Middle East, but these rivers played a crucial role in both military strategy and administration. In the south, the Roman Empire was bounded by the Mediterranean Sea, serving as a vital trade route. These geographic features were central to both the empire's defense and its expansionist policies.