I raised this question because I found the idea of high walls, be they amethyst or opal, to be a grim, prison-like arrangement. However, in the dimension given in Rev 21, though the city is a cube of 1500 miles, the wall of the city only goes up 72 yards:
[Rev 21:15-17 NASB] (15) The one who spoke with me had a gold measuring rod to measure the city, and its gates and its wall. (16) The city is laid out as a square, and its length is as great as the width; and he measured the city with the rod, fifteen hundred miles; its length and width and height are equal. (17) And he measured its wall, seventy-two yards, according to human measurements, which are also angelic measurements.
This answers my original concern which is the prison-like effect of living within walls. I think the walls in such a city are more adornment for her nuptials.
The other 1,499 miles of city would have no such visible structure. The true wall of protection for all 1500 miles is the LORD. This is reflected in this image:
Note: In this image the new Jerusalem is in the sky, before it descends. After it descends it is on earth and in the sky in that its base is on the earth while it extends into heaven. Inside are angels going up and down on ladders (or maybe steps?):
[Gen 28:12, 17 NASB] (12) He had a dream, and behold, a ladder was set on the earth with its top reaching to heaven; and behold, the angels of God were ascending and descending on it. ... (17) He was afraid and said, "How awesome is this place! This is none other than the house of God, and this is the gate of heaven."
There are many such takes on prophecy in the NT that seems to contain a tweak but upon further reflection, contain a deeper truth for those inclined to go treasure seeking.