As you approach the Keep by following the northern road, you’ll come in contact with a small camp first. This camp is set in front of the Sea Keep’s gate. This gate will be closed, and you won’t be able to open it from this side. Luckily for us, we don’t need to have it open. Behind this closed gate is a bridge that leads into the Keep.
Our goal is under the large Keep’s northern bridge – Under Sea Keep’s Towering Fortifications. Drop down from this peak, and find your way under the bridge. Although you can say this is somewhat well hidden, due to the fact that this area is not something you can picture yourself exploring, with the knowledge of its location you can now spot it without difficulty. The Sea Keep’s Skyshard is next to one of the two bridge pillars.
Sand Kingdom Moons - Super Mario Odyssey Power Moons