Jerusalem’s Deep Valley’s
Jerusalem was surrounded on the west, south, and east by deep ravines the which are 200-400 feet deep and therefore made it impossible for an enemy to attack from either these directions. Therefore Herod's Jerusalem was considered unapproachable, except from the north side which was actually protected by the outermost wall which was over 100 feet high and had 90 towers according to Josephus.
The deep valley on the west and the southwest side of the city was called the valley of Hinnom (the abhorred place).
The deep valley on the east side of the city was called the valley of the Kidron, or Jehoshaphat, where the prophet Joel saw a futuristic vision where the nations of the world would be summoned for judgment. The place where these ravines met was called "Enrogel" or The Well of Joab (2 Sam 17:17).
These deep valley’s made the inhabitant’s of Jerusalem to feel safe and secure, as though God Himself were protecting it. It was so secure from an enemy attack that Titus, the Roman General who conquered Jerusalem in 70 A.D. said that "if it had not been for the internal dissensions, the city could never have been taken."