I’m having a hell of a time finding any information about an actual prison or dungeon in Waterdeep. I have a character that’s murdered a citizen and a horse “in self defense.” I’m even looking at the TSR books and can’t find a solid answer. There are lockup cells in every walltower (tower along the city wall), along with local Watch barracks/patrol bases. There's a full-fledged dungeon under Castle Waterdeep. Mentioned in the early Waterdeep FR products and in my 2-part Wizards website article on the City Watch.
— Ed Greenwood (@TheEdVerse) May 2, 2019