You’ll receive The Weight of Chains quest in God of War Ragnarök while on your journey to find the key on the island as soon as you arrive and dock your boat there. You’ll be able to clear away the gilded metal with the nearby explosive pot which will spawn a bunch of wretches. Get killing, but make sure you keep pushing forwards, as there’s a nest at the top of the slope that keeps spawning them. Destroy it.
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Behind the nest is a platform you can climb up. On the far left corner, there’s more debris with explosives next to it. Clearing it gives you access to a path you can climb up on the other side. After clearing out another wretch nest, there’s a gate you can open that will give you a shortcut should you ever need to return.
From there, you’ll find a wheel that lowers an elevator which you can freeze in place by throwing your ax at the exposed gear. This will give you a new path to follow along the climbing route, as you can now head to the right.
This will put you onto the frozen elevator, so recall your ax to send it up. A prompt will then appear to smash through the ground using Circle. Killing the enemy down here will give you the Watchtower key. Take this to the tower above Sindir’s shop right in the middle of the bay. The door will swing open, letting interact with the drum that reveals a hidden location: Lyngbakr Island