I picked one of my favorite spots in Rome - the campidoglio. I started with a bounding box and an oval in the center from cross points. The boxes in the corners changed size based on the desired stairwell width. The inside is composed of diagonal construction lines spaced from the horizontal center line, as well as concentric circles based off of each other's radius.
Bounding box (perpendicular vs. parallelogram)
Size of main circle
Size of inside circles (changing size of cross hatching)
Size of diagonal construction line spacing
Offsetting center point
I learned that it was super easy for everything to get carried away, especially when points "looked" like they were snapping and weren't. It pays to be careful and set everything up, so when you change one thing, it moves seamlessly.