Are God's friends afraid of the wicked charging their city?

No, there is the sound of singing. They are praising God for saving them from Satan. while the city is surrounded  by soldiers and their weapons, something happens to Jesus. Jesus is made KING OF KINGS. He is given His crown by God Himself. All the wicked watch. All those who love Jesus watch. The record books are open for all to see the evidence.

In the sanctuary story of the Day of Atonement, you can learn what happens to the wicked at this time. They do not burn for ever. They become ashes. That is the end. Malachi 4:3 says they are ashes. A kind God does bring things to an end. There is not torture lasting forever. We could not stand watching this. God has no pleasure in torture.

Malachi 4:3 "And ye shall tread down the wicked; for they shall be ashes under the soles of your feet in the day that I shall do [this], saith the LORD of hosts."

Quiz

1. How is Satan bound?

2. How long is Satan bound for?

3. Who is on this earth during this time alive?

4. What does the earth look like during this time?

5. What happens at the end of this time?

6. How healthy do the wicked look when they are raised from the dead?

7. What appears above the city when the army begins to attack?

8. Can you name some people that will be in that wicked army? (see details in the "More Information" section.

9. When Satan and the wicked bow down and say God, the Judge has pronounced the right sentence on them, are they sorry? Can they be good and live in heaven?

10. What do they do that confirms how they would act if taken to heaven?

11. Do the wicked burn forever?

12. What text helps to document your answer?

13. How many does the Bible say will be on Satan's side? (Rev. 20:8)

14. Will the people who worship the beast and receive his mark 666 in their foreheads and right hands be in heaven? (Rev. 20:4)

15. Who is raised from the dead when Jesus comes the 3rd time, bringing the Holy City to set down on earth?

16. Where is the Holy City set down?

17. How many times do the wicked die? (Rev. 20:6)

18. What do God's Bible loving people do during the 1,000 years and where are they?

19. Is Jesus and His people happy to watch the wicked burn up? (Ezekiel 33:11)

20. What is left after the wicked burn up? (Malachi 4:3)

21. How long is Satan loosed? (Rev. 20:3)

More information about the video above the Holy City

I highly recommend reading the Book Great Controversy  chapter 42. God gave details which really help understand the mercy and justice of God in this destruction.

"Above the throne is revealed the cross; and like a panoramic view appear the scenes of Adam's temptation and fall, and the successive steps in the great plan of redemption. The Savior's lowly birth; his early life of simplicity and obedience; his baptism in Jordan; the fast and temptation in the wilderness; his public ministry, unfolding to men Heaven's most precious blessings; the days crowded with deeds of love and mercy, the nights of prayer and watching in the solitude of the mountains; the plottings of envy, hate, and malice which repaid his benefits; the awful, mysterious agony in Gethsemane, beneath the crushing weight of the sins of the whole world; his betrayal into the hands of the murderous mob; the fearful events of that night of horror,--the unresisting prisoner, forsaken by his best-loved disciples, rudely hurried through the streets of Jerusalem; the Son of God exultingly displayed before Annas, arraigned in the high priest's palace, in the judgment hall of Pilate, before the cowardly and cruel Herod, mocked, insulted, tortured, and condemned to die,--all are vividly portrayed. {GC88 666.2} Great Controversy by Ellen White

More

And now before the swaying multitude are revealed the final scenes,--the patient Sufferer treading the path to Calvary; the Prince of Heaven hanging upon the cross; the haughty priests and the jeering rabble deriding his expiring agony; the supernatural darkness; the heaving earth, the rent rocks, the open graves, marking the moment when the world's Redeemer yielded up his life. {GC88 667.1}
The awful spectacle appears just as it was. Satan, his angels, and his subjects have no power to turn from the picture of their own work. Each actor recalls the part which he performed. Herod, who slew the innocent children of Bethlehem that he might destroy the King of Israel; the base Herodias, upon whose guilty soul rests the blood of John the Baptist; the weak, time-serving Pilate; the mocking soldiers; the priests and rulers and the maddened throng who cried, "His blood be on us, and our children!"--all behold the enormity of their guilt. They vainly seek to hide from the divine majesty of His countenance, outshining the glory of the sun, while the redeemed cast their crowns at the Savior's feet, exclaiming, "He died for me!" {GC88 667.2}

Amid the ransomed throng are the apostles of Christ, the heroic Paul, the ardent Peter, the loved and loving John, and their true-hearted brethren, and with them the vast host of martyrs; while outside the walls, with every vile and abominable thing, are those by whom they were persecuted, imprisoned, and slain. There is Nero, that monster of cruelty and vice, beholding the joy and exaltation of those whom he once tortured, and in whose extremist anguish he found Satanic delight. His mother is there to witness the result of her own work; to see how the evil stamp of character transmitted to her son, the passions encouraged and developed by her influence and example, have borne fruit in crimes that caused the world to shudder. 

There are papist priests and prelates, who claimed to be Christ's ambassadors, yet employed the rack, the dungeon, and the stake to control the consciences of his people. There are the proud pontiffs who exalted themselves above God, and presumed to change the law of the Most High. Those pretended fathers of the church have an account to render to God from which they would fain be excused. Too late they are made to see that the Omniscient One is jealous of his law, and that he will in nowise clear the guilty. They learn now that Christ identifies his interest with that of his suffering people; and they feel the force of his own words, "Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me." [MATT. 25:40.] {GC88 668.1}
The whole wicked world stand arraigned at the bar of God, on the charge of high treason against the government of Heaven. They have none to plead their cause; they are without excuse; and the sentence of eternal death is pronounced against them. {GC88 668.2}

...And now Satan bows down, and confesses the justice of his sentence. {GC88 670.1}

Notwithstanding that Satan has been constrained to acknowledge God's justice, and to bow to the supremacy of Christ, his character remains unchanged. The spirit of rebellion, like a mighty torrent, again bursts forth. Filled with frenzy, he determines not to yield the great controversy. The time has come for a last desperate struggle against the King of Heaven. He rushes into the midst of his subjects, and endeavors to inspire them with his own fury, and arouse them to instant battle. But of all the countless millions whom he has allured into rebellion, there are none now to acknowledge his supremacy. His power is at an end. The wicked are filled with the same hatred of God that inspires Satan; but they see that their case is hopeless, that they cannot prevail against Jehovah. Their rage is kindled against Satan and those who have been his agents in deception, and with the fury of demons they turn upon them. {GC88 671.2}

..."The indignation of the Lord is upon all nations, and his fury upon all their armies: he hath utterly destroyed them, he hath delivered them to the slaughter." "Upon the wicked he shall rain quick burning coals, fire and brimstone, and a horrible tempest: this shall be the portion of their cup." [ISA. 9:5; 34:2; PS. 11:6 (MARGIN).] Fire comes down from God out of Heaven. The earth is broken up. The weapons concealed in its depths are drawn forth. Devouring flames burst from every yawning chasm. The very rocks are on fire. The day has come that shall burn as an oven. The elements melt with fervent heat, the earth also, and the works that are therein are burned up. [MAL. 4:1; 2 PET. 3:10.] The earth's surface seems one molten mass,--a vast, seething lake of fire. It is the time of the judgment and perdition of ungodly men,-- "the day of the Lord's vengeance, and the year of recompenses for the controversy of Zion." [ISA. 34:8; PROV. 11:31.] {GC88 672.2}

The wicked receive their recompense in the earth. [ISA. 34:8; PROV. 11:31.] They "shall be stubble; and the day that cometh shall burn them up, saith the Lord of hosts." [MAL. 4:1.] Some are destroyed as in a moment, while others suffer many days. All are punished "according to their deeds." The sins of the righteous having been transferred to Satan, he is made to suffer not only for his own rebellion, but for all the sins which he has caused God's people to commit. His punishment is to be far greater than that of those whom he has deceived. After all have perished who fell by his deceptions, he is still to live and suffer on. In the cleansing flames the wicked are at last destroyed, root and branch,--Satan the root, his followers the branches. The full penalty of the law has been visited; the demands of justice have been met; and Heaven and earth, beholding, declare the righteousness of Jehovah. {GC88 673.1}