Answer:Geographic featured that acted as norther borders of the Roman Empire at its height were: Rhine River, Danube River, Carpathian Mountains, and the Black Sea.
I think it is Christianity
Cliffs are very heavy rocks and when there is too much weight it can cause cracks that weakens the structural integrity which can cause the cliff to either collapse or give way, falling on the ground below which can kill people crush cars or block roads. Hope this helps :)
