"The heavenly messenger measured the holy city and it was twelve thousand furlongs or fifteen hundred miles: the length and the breadth and the height of it were equal, as we have pointed out — a cube. Some have supposed that a literal city must be meant since various measurements are given. However, we must always keep in mind that this city is the bride, the Lamb’s wife! Therefore such dimensions must necessarily have a symbolical and spiritual meaning in relation to the body of Christ! The measurements are in multiples of twelve which are a symbol and signature of God’s purposes in His elect. The number twelve often enters into the symbolism of the book of Revelation — twelve tribes, twelve stars, twelve apostles, twelve angels, twelve foundations, twelve gates. So here the number twelve is found in the measurements of the city: each side is twelve thousand furlongs and the wall is one hundred and forty-four cubits, which is twelve times twelve."
Much more in the link:
https://www.godfire.net/eby/Rev/rev246.html
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"We also learn in the formation of this city of gold and putting nothing in it but the best, that we must begin with the foundation just as every builder does. Paul tells us about the foundation, that it is Jesus Christ. In another sense, in the Revelation, it is the apostles of the Lamb. Now how can both be true? The answer is not difficult to find. When the life, nature, faith, wisdom, and power of the Lord is laid in our lives the foundation is laid in us — that is Jesus Christ. Paul well knew his calling. It had been from heaven and after God revealed it to him, Paul didn’t go and ask the other apostles what they thought about it, but got busy right away with getting the foundation laid first in his own life, and then in those he ministered to. Thus he said, "I am determined to know nothing among you save Jesus Christ and Him crucified." And that is the foundation for every saint of God who will press on to the glory and majesty of the holy city, the precious stone that has been laid in the earth, the tried stone, the sure foundation that will never fail. The dread infirmity of the Lord’s people is just this — that not only are they oft times building with inferior materials of wood, hay and stubble, but many are not building on the true and only foundation of JESUS CHRIST. The foundation is not a generic Christ, nor some cosmic Christ, nor the Christ principle — it is JESUS CHRIST and none other. It was Paul’s lot to lay the foundation in men which is nothing else but Jesus Christ the Lord!"