War is the key factor because this is the main reason why people in ancient times built on hills. A high hill with steep slopes and a defendable water source near a valley where you could grow crops made it difficult for enemy armies to conquer your city.
Walls–Ancient people built on hills, not in the valley, because they could defend themselves better on high ground. On top of the hills they added high and large stone walls to make it even harder for their enemies to get into the city to kill or capture them or their families.
