Yeah it's unreal. And the result is so well-proportioned and 3-dimensional as well. I wonder how much of it was in his mind before he started, and how much was about figuring out what would fit along the way.
I can't remember the source, but I remember learning that his specialty was 5-point perspective and he could keep most of the drawing visualized in his mind before he even started. But he also improvised a lot and made decisions on the fly.
Unreal is definitely the right word, similar to watching Michael Jordan at his best.