When you first encounter a snake, it appears to appear out of nowhere. On your way to the Ashina Castle Fortress area, you need to walk through the Underground Bridge Valley. Here you will come across a giant white snake.
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To get the Young Lord's Bell, you must find an old woman in red rags in the Ashina Castle grounds. It is located on the first floor of a ruined house right in front of the path that leads to the Giant in the Blocks.