Revelation 21:12-21 gives a detailed description of the city of God, the New Jerusalem. This is the place Jesus went to prepare (John 14:1-3). This is the city that comes down out of heaven from God. It is the eternal dwelling of God with His people. The number twelve is repetitious with meaning in the city of God. There are twelve gates with twelve angels. There are twelve foundations of the city, with walls filled with twelve types of precious stones.
The number twelve represents the redeemed people of God. There were twelve tribes of Israel in the Old Testament. We see the twelve apostles of the Lamb in the New Testament. The number represents all of God's people. The inhabitants of the city are God's covenant people. They are the redeemed of the Lord.
The twelve gates represent the entrance to the city by all of God's people. The twelve angels represent the heavenly host who ministered to those who are heirs of salvation. There is no need for the gates to be closed. That's because all that have done evil to them have been cast into the lake of fire.
Twelve precious stones or jewels compose the walls of the city. They represent the twelve stones in the breastplate of the High Priest of Israel in the Old Testament. He foreshadowed our Lord Jesus Christ, the great High Priest of the everlasting covenant (Hebrews 13:20-21). That testifies to our redemption from sin by the blood of the Lamb (John 1:29).
