Uncover a Lost Secret of Edinburgh Castle

don’t start this early in the morning as you can’t access the castle(this may have been because it is late July and they are setting up for the tattoo?) lady stairs close/writers museum we couldn’t find the statue with the open book. at advocates close (found several Latin quotes but perhaps we didn’t go far enough) none matched the answer. it then went back to lady stairs close (without directions) and whilst we found a Robert Louis Stevenson quote it wasn’t the one with the answer. Tron Kirk wasn’t open and the only plaques outside above the door had Roman numeral dates different from the answer. holyrood palace wasn’t open so couldn’t find the last clue perhaps it would have been a more successful walk in the afternoon? the story and “did you know” history was enjoyable but perhaps the clues need reviewing or updating