And they came up on the broad plain of the earth and SURROUNDED
After the terrible judgments of the tribulation period, the earth is very much different geographically. The implications of the earth shaking in Revelation 16:20 would seem to be confirmed by this portion of scripture. The earth is one broad plain.
If one looks at the spiritual meanings of this portion, we find that there are no mountains to hide behind. This may be another way of saying that all of the evil contained in the sinful nature of mankind is being exposed. This evil, of necessity, was kept hidden during the millennial period, but now with an emboldening from the release of the devil, evil has become an open, hostile threat to the SAINTS. One can look at it from a different perspective and realize that this may be God's way to gather all this evil into one focal point (easier to devour).
The CAMP of the SAINTS is collocated at the BELOVED CITY. Most scholars agree that this is Jerusalem. Prophetic scripture indicates that Christ will rule from there (Isaiah 24:16-23; Micah 4:6-8; and Zechariah 14:9-21) during the 1,000 years. The original Greek word for "CAMP" implies a military connotation, and this would be fitting for the "armies of heaven" (Revelation 19:14). If you have served in the military, you understand that "CAMPS" are temporary, and so too here, this CAMP is temporary. Our new home still awaits (Revelation 21:1-2-8).
If you consider that the Revelation discussion following the tribulation is mostly spiritual in context, then the CAMP of the SAINT and the BELOVED CITY could be thought of as the collection of all true believers remaining into the millennium. It is the place where the RIGHTEOUS and HOLY ones are gathered.
and FIRE CAME DOWN FROM HEAVEN and DEVOURED them.
Those who believed the Devil's arguments are consumed by FIRE FROM HEAVEN. It was their "unbelief" that Jesus, who had ruled them for 1,000 years, was their Savior and thus was much better than the devil. Those who are born during the millennial period had their names recorded in the book of life, and their "unbelief" erased their names from that book. It also killed them (John 8:21-30) eternally (verse 15).
From another view, this is the way we are shown that the evil (more properly, the knowledge of evil that was released in the Garden of Eden, Genesis 2:15-17) has been totally wiped off of the earth.
This is a Godly judgment, as the FIRE came down from HEAVEN. Recall the two other times this happened: Sodom and Gomorrah (Genesis 19:24-29) and the emissaries of Ahaziah (2 Kings 1:9-17). The reader can determine what 'evils" were removed in those two instances.
One last thing, contrast this DEVOURING with the removal of those who took the mark of the beast (Revelation 19:21). It wasn't FIRE then, but rather a "SWORD". Why?