I don't think it's that hard to stumble across something that only requires you to use the submerging ability of Figaro castle to find. I found it back as a kid with nothing to go on but the map that came with the game. I doubt I was even looking for the Ancient Castle when I found it thogh. I wouldn't call it luck. Just natural curiosity/inclination to play around with stuff.

I don't even know what you're talking about in regard to the "hint," but seems like the hint isn't even necessary to me.