were there stairs and an the hylian shield carved into the wall? by saas ko'sah shrine?That thing has like a million entries, but there's one at the northwest side of the castle, close to one of the pillars. From there you can enter the prision where there's like a bunch of powerful enemies that can 1 shot you, i just froze them with my ice arrows and continue my way until i got locked up with a (Rather easy) mini boss. Defeat it and you'll get what you're looking for plus some really good weapons.