Why will the New Jerusalem have walls?

If walls are for protecting, why does the Bible say the New Jerusalem will have walls around it? Is Hell located near the New Heaven and New Earth?

The complete fulfillment of all God has promised to us since the beginning, comes in the place of the New Heaven and New Earth.

Revelation Chapter 21 begins with John, atop a mountain, seeing the city from a distance describing measurements and dimensions of the Holy City:

REV. 21:9 Then one of the seven angels who had the seven bowls full of the seven last plagues came and spoke with me, saying, “Come here, I will show you the bride, the wife of the Lamb.”
REV. 21:10 And he carried me away in the Spirit to a great and high mountain, and showed me the holy city, Jerusalem, coming down out of heaven from God,
REV. 21:11 having the glory of God. Her brilliance was like a very costly stone, as a stone of crystal-clear jasper.
REV. 21:12 It had a great and high wall, with twelve gates, and at the gates twelve angels; and names were written on them, which are the names of the twelve tribes of the sons of Israel.
REV. 21:13 There were three gates on the east and three gates on the north and three gates on the south and three gates on the west.
REV. 21:14 And the wall of the city had twelve foundation stones, and on them were the twelve names of the twelve apostles of the Lamb.

Curiously, the city is surrounded by a high wall with three gates on each side for a total of twelve gates. Normally, walls are for defense, which may cause us to wonder why this city needs defenses? Later in v.25 we will learn that the gates never close, so clearly no one is worried about an attack, therefore, these walls and gates are not defensive fortifications, they are memorials remembering God’s work in the prior Creation.

Notice the gates are arranged in threes for a total of 12 entry points; these numbers are significant. The number 3 is the number for the Godhead (Father, Spirit, and Son), which reminds us that the architect behind everything is God, from the Creation account in Genesis to the creation of the New Heaven and Earth.

All the work was God’s alone and which He accomplished through the nation of Israel. The number 12 represents God ruling through people, specifically through the sons of Jacob. So the gates will forever remind us that the Lord brought us into this city by working through a family called Israel.

The foundation of the walls will contain 12 different layers of precious stones representing the 12 men who Jesus called into ministry while He walked the earth. Similarly, John says the foundation will memorialize the role of the 12 Apostles. The Apostles were the men God used to establish the Church and the opportunity for Gentiles to enter into the city as well. The memorialization of the two groups together reinforces that Israel and the Church are distinct and yet work together in God’s plan. Paul explains this relationship in Ephesians 2:

EPH. 2:12 remember that you [Gentiles] were at that time separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world.
EPH. 2:13 But now in Christ Jesus you who formerly were far off have been brought near by the blood of Christ.
EPH. 2:14 For He Himself is our peace, who made both groups into one and broke down the barrier of the dividing wall,
EPH. 2:15 by abolishing in His flesh the enmity, which is the Law of commandments contained in ordinances, so that in Himself He might make the two into one new man, thus establishing peace,
EPH. 2:16 and might reconcile them both in one body to God through the cross, by it having put to death the enmity.
EPH. 2:17 AND HE CAME AND PREACHED PEACE TO YOU WHO WERE FAR AWAY, AND PEACE TO THOSE WHO WERE NEAR;
EPH. 2:18 for through Him we both have our access in one Spirit to the Father.
EPH. 2:19 So then you are no longer strangers and aliens, but you are fellow citizens with the saints, and are of God’s household,
EPH. 2:20 having been built on the foundation of the apostles and prophets, Christ Jesus Himself being the corner stone,
EPH. 2:21 in whom the whole building, being fitted together, is growing into a holy temple in the Lord,

The two groups described in Ephesians were once separated by a law that God gave to Israel, however, now that Christ has done away with the Law of Moses, He has broken down the dividing wall allowing both groups to join in salvation. This new combined family is built on a foundation of the apostles and prophets, Paul says, meaning their roles in bringing God’s revelation. Thus, the New Heaven and Earth will be a Holy City constructed to remind us of Israel’s role in allowing all people to enter God’s grace.

Further, we must also take into consideration the clues we see in Revelation 14 concerning the Lake of Fire's location:

Rev. 14:9 Then another angel, a third one, followed them, saying with a loud voice, “If anyone worships the beast and his image, and receives a mark on his forehead or on his hand,
Rev. 14:10 he also will drink of the wine of the wrath of God, which is mixed in full strength in the cup of His anger; and he will be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb.

Notice the location of the place of eternal torment is said to be "in the presence of the holy angels and in the presence of the Lamb.” This description would imply that the Lake of Fire will be located in the heavenly places, not in the physical realm. Therefore, its existence is not impacted by the destruction of the first heavens and earth following the 1,000-year kingdom, since it is not a part of the created universe.