Steel Sword (Advanced): Return To The Watchtower

Head to the Ruined Watchtower at the northwestern end of the huge lake, where you found one of the original diagrams for the silver sword. On this occasion, you’re after steel sword plans; enter via the main entrance, and climb this upper wall.

Then it’s simply a quick dash across the remains of the wooden platforms to the chest directly above the arched entrance. Pry it open and take the last enhanced diagram you need.