One sentence on what the product does and who it is for. One more on why it was hard.
Two or three paragraphs. Describe the state of things before you arrived: the constraint, the cost of leaving it alone, and who was feeling the pain. Concrete numbers here do more work than adjectives.
A short lead-in, then the three decisions below. This is the section a hiring manager actually reads — each decision should name the alternative you rejected and why.
Walk through the request path in plain language. Service boundaries, where state lives, what happens when something fails.
Close with the result and one honest note on what you would do differently. The reflection is what separates a case study from a changelog.