Like most other Assassin's Creed Shadows Castles, you need to eliminate all of the Samurai within the castle itself to then gain access to the legendary loot chest. You can return to the castles at any time and leave at any point to refuel, also if you're struggling to take down a Samurai we recommend switching characters to Yasuke.

I used to be the sentimental type in video games. For instance, if a certain character offered you a weapon—a keepsake—I would hang on to that weapon for as long as possible. Something like Naoe’s starting katana would be the kind of thing that I’d stubbornly hold onto for far too long.

To claim the unique gear available from forts and castles in Assassin's Creed Shadows, Yasuke and Naoe must first defeat the Samurai Daisho.