"The ruined city lies desolate; the entrance to every house is barred. " NIV translation
The "ruined city" speaks of Jerusalem and the fact that it is worthless. What is meant by calling the city of God worthless? Even God's own city and its people have rejected their Messiah and because of that fact they are no longer of use to Him. Every house is barred speaks of the fact that all of the people (or a very great majority) have surrendered to the deception of the devil. The city of Jerusalem was set aside by God to be a shining example of His presence in the world but, by rejecting the grace of Jesus Christ, it can no longer serve that purpose.
God's people have been removed from the earth and so the joy has turned to sorrow and gloom. This speaks of what is commonly called the "rapture" or the calling up of Christians to be with the Lord and escape this judgment.
The physical Old City of Jerusalem has many gates but, here, Isaiah speaks of only one gate. The gate being "battered to pieces" speaks of the rejection of the fact that Jesus is the Son of God and the payment for our sins. Jesus told the disciples that He was the only way and men have rejected Him as the gate.
"In the streets they cry out for wine; all joy turns to gloom, all joyful sounds are banished from the earth. The city is left in ruins, its gate is battered to pieces." NIV translation
At this time, the misery will become so great that people will cry out for alcohol and drugs to help them to escape the world around them.
"So will it be on the earth and among the nations, as when an olive tree is beaten, or as when gleanings are left after a grape harvest." NIV translation
This verse speaks of the remnant of Christians that is left behind and is what John saw as the 144,000 of Revelation 7. Throughout the Bible, the olive and the olive tree speak of righteousness and are a picture of the righteousness obtained through faith in Jesus Christ. The "gleanings" speak of the fact that not all of the fruit is taken at this first harvest.