Revelation 16:12-16
The sixth angel poured out his bowl on the great river, the Euphrates; and its water was dried up, so that the way would be prepared for the kings from the east.

And I saw coming out of the mouth of the dragon and out of the mouth of the beast and out of the mouth of the false prophet, three unclean spirits like frogs; for they are spirits of demons, performing signs, which go out to the kings of the whole world, to gather them together for the war of the great day of God, the Almighty. ("Behold, I am coming like a thief. Blessed is the one who stays awake and keeps his clothes, so that he will not walk about naked and men will not see his shame.") And they gathered them together to the place which in Hebrew is called Har-Magedon.

Har= a Hebrew word meaning hill/mageddon derive  from the name of a plain called (megiddo.) 

Whosoever controled the valley of  Megiddo was the one who had the power to win any war. Pharaoh Thutmose III (15th century BC), stated.   "The capture of Megiddo is the capture of a thousand cities." 

Why would this ancient city be so important in war?

Answer:  Megiddo sat near the end of a level pass, rich in springs, which slices through the hills from the coast to the Jezreel Plain.

Ancient travelers between Egypt and Damascus preferred the Jezreel plain: It gave them a flat stretch across most of the country, whether they were aiming for Damascus, Acco, Beth Shean and the King's Highway, or Samaria.

Archaeologists count up to 22 major strata, from the Chalcolithic period to the Persian (4th century BC). These include Solomon's Megiddo (1 Kings 9:15), as well as Ahab's. The Assyrians executed Judah's king Josiah here. The prophet Zechariah referred to as the "mourning of Hadadrimon in the plain of Megiddo" (Zechariah 12:11). As prophetic tradition, this reference lies behind the vision in Revelation concerning Armageddon.

Three major attractions at Megiddo are:  The view over the Jezreel Plain, host of many battles,  the cultic area, the water shaft.  In the future this is were the state of the world, the last 111 World War will be played out.  Winner takes all.