I suspect that it's like another underground city that was described once upon a time by ICE.
What is shown are the main levels, there's probably more levels in between, but they are living quarters, storerooms and all.
The towers are at 6 miles in height according to the text. so from the Level 1 to the top of the towers it's 6 miles high.
The crater fields seems to be between 4 and 5 miles up.
As for the connection between each main level the drawings are not clear but it seems that it's an elevator system.
Now that I look at the thing I find all the things over exagerated, so I suspect that a great resizing is in order...