What counts as a castle — the rule, written down

The most-repeated question in the Hacker News thread was never really about a missing castle: it was &ldquo;what is your definition, and why is it applied so inconsistently?&rdquo; The atlas had no answer anywhere on the site. It does now — the About page states the rule plainly. Castle & château is a fortified residence, fortress & fort is built for a garrison rather than a household, palace is a…