You will encounter a locked Watchtower in Svartalfheim while completing the Weight of Chains favor in God of War: Ragnarok.
Your objective will be to enter the Watchtower by finding its key. However, the only clue you will be given is that the key is located somewhere in the Bay of Bounty of Svartalfheim.
Weight of Chains is an important side quest to complete. You will be able to access a new area to continue your search for more collectibles. That as well as freeing the imprisoned creature.
The following guide will tell you where to find the Watchtower key in God of War: Ragnarok.
Find the Watchtower key in the Bay of Bounty
The Watchtower key can be found on Lyngbakr Island which is beside a big geyser. You can use your boat docked below the Watchtower and head northeast to dock on the island.
Now clear out your path by destroying fire pots and continue along this route. Climb up the ledge and clear out some more fire pots blocking your path.
Make your way across by climbing and hanging from the ledge. Interact with the spin wheel in front to drop the elevator and freeze the gears in position using the Leviathan Axe.
Hang by the ledge again and this time move further along to your right to climb onto the elevator. Unfreeze the gears to make the elevator rise once again.
Now break through the platform below to enter a room with a chest. Clear out the room and open the chest to acquire the key to the locked Watchtower.