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."
All of the rubbish from the entire city of Jerusalem was dumped and burnt here.
Otherwise called "the valley of the son of Hinnom," or "the valley of Benhinnom"; a deep and narrow ravine with steep, rocky sides to the S and W of Jerusalem, separating Mt. Zion to the N from the "Hill of Evil Counsel," and the sloping rocky plateau of the "valley of Rephaim" to the S. The earliest mention of the valley of Hinnom is in Josh 15:8; 18:16, where the boundary line between the tribes of Judah and Benjamin is described as passing along the bed of the ravine. On the southern brow, overlooking the valley at its eastern extremity, Solomon erected high places for Molech (1 Kings 11:7), whose horrid rites were revived from time to time in the same vicinity by the later idolatrous kings. Ahaz and Manasseh made their children "pass through the fire" in this valley (2 Kings 16:3; 2 Chron 28:3; 33:6), and the fiendish custom of infant sacrifice to the fire-gods seems to have been kept up in Topheth at its southeast extremity for a considerable period (Jer 7:31; 2 Kings 23:10). To put an end to these abominations the place was polluted by Josiah, who rendered it ceremonially unclean by spreading over it human bones and other corruptions (2 Kings 23:10,13-14; 2 Chron 34:3-5). From that time it appears to have become the common cesspool of the city, into which its sewage was conducted to be carried off by the waters of the Kidron, as well as a laystall, where all its solid filth was collected. From its ceremonial defilement and from the detested and abominable fire of Molech, if not from the supposed everburning funeral piles, the later Jews applied the name of this valley Ge Hinnom, "Gehenna," to denote the place of eternal torment. The name by which it is now known is Wadi Jehennam, or Wadi er Rubeb. See Gehenna; Hell.
THE VALLEY OF HINNOM (GEHENNA)
All the rubbish of Jerusalem was burnt here. Its interesting to note that Aceldama (Field of Blood) was here in this valley.
KID'RON (kid'ron; "turbid, dusky, gloomy"; Grk. Kedron; "Cedron," John 18:1, KJV). The brook that flows through the valley of Jehoshaphat. The name was also applied to its bed, the valley of Kidron. It is thus described by Smith (Hist. Geog., p. 511): "To the north of Jerusalem begins the torrent-bed of the Kidron. It sweeps past the Temple Mount, past what were afterward Calvary and Gethsemane. It leaves the Mount of Olives and Bethany to the left, Bethlehem far to the right. It plunges down among the bare terraces, precipices, and crags of the wilderness of Judea-the wilderness of the scapegoat. So barren and blistered, so furnace-like does it [the valley] become as it drops below the level of the sea, that it takes the name of Wady-en-Nar or the Fire Wady. At last its dreary course brings it to the precipices above the Dead Sea, into which it shoots its scanty winter waters; but all summer it is dry." The valley is only 20 miles long but has a descent of 3,912 feet. The place where it enters the Jordan is a narrow gorge about 1,200 feet deep.
The Kidron was the brook crossed by David when he fled from Absalom (2 Sam 15:23). Solomon fixed it as the limit of Shimei's walks (1 Kings 2:37); beside it Asa destroyed and burned his mother's idol, or Asherah (15:13); here Athaliah was executed (Josephus, Ant. 9.7.3; cf. 2 Kings 11:16). It then became the regular receptacle for the impurities and abominations of the idol worship when removed from the Temple and destroyed by the adherents of Jehovah (23:4,6,12; 29:16; 30:14); and in the time of Josiah this valley was the common cemetery of Jerusalem (2 Kings 23:6; Jer 26:23; 31:40).