Should the keep, towers and enclosed lands ever be restored to function, it would be practically self-sustaining - but, only if you know what you're doing from the outset and, none of Harrenhal's lords since Harren himself have had the ties, education, means or opportunity to get close to that. No house has ever successfully attempted to fully restore it, though most of its inhabitants have moved in with the firm intention of at least trying to fix it up to be a little better than they found it.
Awesome, but Impractical: Harrenhal is so massive (and rundown) that it's downright impossible to properly garrison or maintain in the classic White Elephant way.
