Your work throughout this challenge is very, very well done. I do have a couple things to draw your attention to, but all in all this critique is going to be quite brief as it's one of many I have to get through before I can get to sleep.

So, as I mentioned, overall you've done an excellent job. You're drawing ellipses that are extremely smooth and confident, your lines are cleanly ghosted, and you're extremely thorough in error checking. There are just two issues I want to call out, one in each section of the exercise.

Firstly, in your cylinders around arbitrary minor axes, while you had started out for a while just drawing cylinders with the same sort of foreshortening, I was very pleased to see that you started changing it up further into the set. This is important, because it allowed me to identify an area of inconsistency in how your cylinders were drawn.

Cylinders - and really every 3D form - experience two kinds of shifts, when comparing one end of the form to the other. In this case, comparing the ellipses on either end, we'll notice that they differ in overall scale, as well as in their relative width (the ellipse's degree). As we move farther back, the scale of the ellipse gets smaller, and the degree of the ellipse gets wider. You've adhered to this pretty well throughout your cylinders, so that's fine.

The issue is however that the amount of each one's shift is actually related. Both of these serve as visual cues as to the distance between the ends of the form. If there's a big shift, then there's a lot of distance and the cylinder is relatively longer. If there's a minimal shift, the cylinder is shorter. If however there's a big shift in one of these properties, but a small shift in the other, then we get an inconsistency.

Looking at cylinder #88, we can see that here you've maintained a pretty minimal shift in degree, but a significant shift in scale. Looking at the degree of the ellipses we get the impression of a small distance between them, and therefore a shorter cylinder. Looking at the scale shift however, we're given the impression of a much larger distance between them, and therefore a longer cylinder. The viewer is given two different, conflicting pieces of information, and that makes the cylinder read as being a little "off", though they won't necessarily know why.

Moving onto the cylinders in boxes, the issue I noticed here was that you weren't consistently drawing the ellipses such that they touched all four corners of the plane enclosing them. This exercise is actually more about the box than the cylinder, and so this mistake actually undermined the exercise somewhat.

This exercise is all about learning to construct boxes that are roughly square on either side by eye, or by instinct. We achieve this by taking the concept of the line extensions from the box challenge (which helped us improve our instincts for keeping the convergences more consistent, and therefore drawing lines that are parallel in 3D space), and adding three additional lines per ellipse.

The ellipses introduce the minor axis and the two lines that pass through the contact points, but these lines will only align to the box's own vanishing points when the ellipse itself represents a circle that rests on the surface of the box. If the ellipse represents a circle, then the plane that encloses that ellipse will itself represent a square. If however we don't draw the ellipse to fit up against all edges of the plane, then this kind of falls apart.

Just like in the box challenge, as we repeat this exercise and check our errors after the fact, we can make adjustments to how we're drawing our boxes to improve the chances of our lines all coming together towards the same vanishing points.

Now, while you made this mistake and it largely undermined the efficacy of the exercise, I do still see improvement over the set. It would have been more significant had you followed the instructions more directly, but alas. I won't be making you redo it, just be sure to continue practicing this sort of thing here and there as part of your warmups.

I'll go ahead and mark this challenge as complete.