Put category folders in the URL only when they reflect the site’s real structure, and stay flat when they do not, because neither form wins by default. Folders belong in the URL when they signal genuine topical grouping that helps users and crawlers understand how the site is organized, and a flat structure suits small or non-hierarchical sites where there is no real organization to express. The pivot is whether the folder reflects how the site is actually structured rather than being an SEO trick bolted onto the path. Match the URL to the real organization and the question answers itself.
Folders earn their place when the grouping is true. On a site with distinct sections, putting a page inside the folder for its category communicates that grouping in the URL itself, which helps users understand where they are and helps crawlers see how content clusters together. The path becomes a small map of the site. When the categories are real and meaningful, that hierarchy is informative, and stripping it out to force everything flat would discard useful structure for no gain.
A flat structure earns its place when there is no real hierarchy to reflect. A small site, or one where pages do not divide cleanly into categories, gains nothing from inventing folder levels just to look organized. Manufacturing depth, stacking folders the content does not actually require, adds length and false structure without communicating anything true. For those sites, a clean flat URL is the honest and simpler choice.
The mistake on both poles is treating URL depth as a ranking dial. Flat URLs do not inherently rank better, and folders are not inherently good for SEO; the value is in accuracy, in the URL telling the truth about how the site is arranged. A folder that mirrors a genuine section helps; a folder invented to seem structured does not. Likewise a flat URL fits a flat site and misfits a deeply organized one.
When you set up your URLs, look at how your site is actually organized and let the path follow it: use category folders where they reflect real, meaningful groupings, and keep things flat where no genuine hierarchy exists. Structure the URL to match reality rather than to chase a depth-based ranking belief.