The valley of the mountains
In the prophecy of Zechariah 14, all nations gather against Jerusalem to battle.
Isaiah said, the mountain of the LORD’s house will be established in the top of the mountains, and will be exalted above the hills. [Isaiah 2:1-3] Isaiah’s prophecy was fulfilled, and Jerusalem was raised up, when Jesus ascended to heaven, after he rose from the grave. Since then, the Jerusalem of prophecy is the heavenly city, rather than the earthly one.
In Zechariah 14:3, the Lord goes forth, and fights against the nations which come against Jerusalem, as he fought “in the day of battle.” Zechariah’s prophecy applies to the heavenly city. The Lord did not fight against the enemies of the Jews, in 70 AD, when Jerusalem was besieged by the Roman armies.
In Zechariah’s prophecy Jerusalem represents the saints, and Zechariah 14:1-5 parallels Revelation 20:8-9, where deceived people from all parts of the earth come against the camp of the saints, and the beloved city.
In Zechariah 14:4, the Mount of Olives cleaves in the midst, and half of the mountain moves to the north, and half of it moves to the south, forming a wide valley. Mountains in prophecy represent revelations of God, and prophecies, and promises, and the Mount of Olives represents the Olivet discourse of Jesus.
Then shall the LORD go forth, and fight against those nations, as when he fought in the day of battle.
And his feet shall stand in that day upon the mount of Olives, which is before Jerusalem on the east, and the mount of Olives shall cleave in the midst thereof toward the east and toward the west, and there shall be a very great valley; and half of the mountain shall remove toward the north, and half of it toward the south.
There are two opposite schools of interpretation of the Olivet discourse: preterism, and dispensationalism. Preterism says that the Olivet discourse applies to the events of 70 AD, when the earthly Jerusalem was destroyed by the Romans. Dispensationalism says that the things foretold by Jesus in the Olivet discourse will occur in a future seven year tribulation. Those two opposite schools of interpretation are pictured by the two halves of the Mount of Olives that are displaced from their positions, moving to the north and to the south, forming a valley between.
And ye shall flee to the valley of the mountains; for the valley of the mountains shall reach unto Azal: yea, ye shall flee, like as ye fled from before the earthquake in the days of Uzziah king of Judah: and the LORD my God shall come, and all the saints with thee.
Zechariah said, flee to the valley between. I suggest that the valley represents the judgment that occurs in the age of the church, the period between the destruction of Jerusalem in the first century, and the end of the age, when Christ returns.
In several prophecies, valleys represent a judgment, as shown in this post. Similarly, Zechariah’s valley of the mountains, an invisible, spiritual valley through the Mount of Olives, represents the judgment occurring in present age, the age of the church. Those who seek the kingdom of God are judged in the present age. The judgment upon the church is represented by Zechariah’s “valley of the mountains.” The ones who are accounted worthy are exempt from judgment in the age to come; I think that is why Zechariah said “ye shall flee to the valley of the mountains; for the valley of the mountains shall reach unto Azal.” No one knows where Azal is located; likewise, no one knows how long the present age will last.
An ancient translation suggests that the meaning of Azal is “a narrow place;” thus, “for the valley of the mountains shall extend to narrowness,” that is “to a narrow place.” 
In the Olivet discourse, Jesus said: “When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:) Then let them which be in Judaea flee into the mountains.” [Matthew 24:15-16]
The mountains that Jesus meant, I think, are symbolic of the promises of God to the saints.
Jacob said, when he blessed his son Joseph, “the blessings of thy father have prevailed above the blessings of my progenitors unto the utmost bound of the everlasting hills.” [Genesis 49:26] The “utmost bound” alludes to their height, suggesting they are lofty, or spiritual, and “everlasting” means eternal. Thus, mountains represent spiritual things, that the saints are to flee to.
Jesus said, “pray ye that your flight be not in the winter, neither on the sabbath day.” [Matthew 24:20] To have to flee on the sabbath day could mean that one has to seek those things, at a time when the saints have come into their promised rest. Similarly, to have to flee in the winter, which follows the time of harvest in the fall, suggests that one has missed out on the harvest, when those who were ready have entered into the kingdom. In the parable of the 10 virgins, the 5 who were ready went in, and the others missed out.
For Christians, Jesus’ warning is to seek the spiritual things that mountains represent; the valley in Zechariah’s prophecy represents the judgment of the saints now, in the present age.
1. Charles Henry H. Wright. Zechariah and his prophecies. 1879. p. 476. note.