“New Jerusalem” is only mentioned in Revelation:
The one who conquers, I will make him a pillar in the temple of my God. Never shall he go out of it, and I will write on him the name of my God, and the name of the city of my God, the new Jerusalem, which comes down from my God out of heaven, and my own new name. (Rev 3:12)
And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. (Rev 21:2)
It is fairly obvious that it is not a literal city. We are told in Revelation 21:16 that the length, breadth and height of the city are all 12,000 stadia, or 2,200 kilometre. The length of the side of the city, if it were literal, would stretch from Jerusalem to Rome, its area would be more than 100 times the whole area of Israel, and its height would be about 200 times the height at which commercial airliners fly. The angel told John it was a figurative city when he said, “Come, I will show you the Bride, the wife of the Lamb, … and [he] showed me the holy city Jerusalem.” (Revelation 21:9-10) So, New Jerusalem is a symbolic representation of the true church, which comprises followers of Jesus Christ.
Why does the new Jerusalem needs walls and gates
As said in the answer, the dimensions of the walls make it highly likely they are symbolic. Similarly, the number of gates was symbolic, and the twelve gates were representing the twelve tribes of Israel: presumably showing that the true believers were following on from and were part of spiritual Israel.
In the old Testament and also in Revelations, the last chapter, we are told that God will create a NEW EARTH and a NEW HEAVEN and the former will be no more.
The New Jerusalem will be on God’s Holy Mountain I suppose on the new earth considering Scripture says the former will be no more.
It makes sense, in Revelations two prophets will be slain and lie the streets of that great city where our Lord was also crucified, why would God put a temple, or anything on the ground his son was murdered? I just don’t think any father woulld build even a shack on top of the place their son was brutally murdered, do you?
Jerusalem was God’s chosen city. The temple had already been built there before, and was already promised to be built there again.
However, 2 Peter 3:13 makes it a little more clear what the new heavens and the new earth mean: they are a place where righteousness dwells, where the right thing is done rather than the wrong thing. As said in the answer, the New Jerusalem is symbolic of the followers of Jesus. Setting it up is in fact a memorial that Jesus did do the right thing and that his death led to life for many. While we can’t always judge God’s actions by what we think we would do, I think most people would be happy to build a memorial for their son showing how good he was and what he achieved on the place where their son was killed.