Captain's Key location and locked door
The Captain's Key is on the ground in the right-hand corner of an underground room in the Fortress, reached by following the route from the Chapel Lantern. It opens the captain's quarters near the Traverse Lantern, opposite the cannon area, where a chest contains a Ferrofluid Flask Power Upgrade.
Choose your starting Lantern
Use Fortress > Chapel as the starting point for the underground pickup route, then return to Fortress > Traverse to use the key. Rest before setting out if you need Health or Nephtoglobin. Resting restores both and revives regular enemies, so prepare to clear previously explored areas again.
Follow the Chapel route to the key
- Follow the underground route from Fortress > Chapel until you reach the sewer room with the pole jump.
- Complete the pole jump to reach the part of the sewer room with a door on the left.
- Go through the left-hand door to reach the staircase into the lower area.
- Descend that staircase to reach the area containing the room with the large gate.
- Take the descending flight of stairs opposite the room with the large gate to continue toward the key.
- Continue into the room at the end of this route to reach the key's pickup location.
- Pick up the Captain's Key from the ground in the room's right-hand corner so you can unlock the quarters near Traverse.
Open the captain's quarters near Traverse
- Return to Fortress > Traverse to reach the Lantern beside the area containing the locked building.
- Face away from the cannon area at the Traverse Lantern to locate the building opened by the Captain's Key.
- Use the Captain's Key on the building's door to enter the captain's quarters.
- Open the chest inside the building to collect the Ferrofluid Flask Power Upgrade.
Recognize the other underground branches
Turning right at the bottom of the initial stairs leads into a room that locks you in with enemies, with an explosive chest containing a Quintessential Blade Material at the back. That room is a separate detour. For the Captain's Key, take the descending stairs opposite the room with the large gate and continue to the room at the end.
The Fortress Armoury Key opens an underground door reached from Chapel, with another Ferrofluid Flask Power Upgrade inside. The Captain's Key opens the building near Traverse, opposite the cannons. Check which door you are trying to open before retracing the underground route.
Apply the upgrade and plan a return
Apply the Ferrofluid Flask Power Upgrade at a Lantern to improve your flask's healing. Finish this detour before continuing beyond the Fortress's High Tower if you want the upgrade during your current visit. You can also revisit the Fortress after the ending to collect missed items.