Ultimate Key Door Locations - Dragon Quest 6
The Ultimate Key in Dragon Quest 6 can be obtained immediately after you get the Lorelei's Harp item from the mermaid north of Port Haven. You'll obtain this item about half way through the game after completing the events that take place in Pescado; check out Chapter 7 of my Walkthrough for more information. What Lorelei's Harp effectively does is allows your ship to travel underwater.
Use Lorelei's Harp near Pescado and travel a tad bit north of the town while underwater to discover a sunken ship on the seabed (pictured above). Inside of this ship all you must do is open the treasure chest that contains the Ultimate Key and it will be yours.
If you've ever played a Dragon Quest game before you probably already know what this key does. It's a reoccuring item throughout the series that typically allows you to open many optional doors throughout the game and obtain some really good items. This key is also used a few times to progress through the game, like at the Tower of Pegasus.
Below are all of the optional locations for you to use the Ultimate Key along with a little bit of information about how to reach each location and what you can find there.
#1 Somnia Castle (Dream World)
Items: Boxer Shorts & Mini Medal
After entering the castle while still on the first floor go to the northeastern most part of the area to find a stairwell down. In this room you'll find a locked door as well as two treasure chests (pictured above). This location has treasure chests in both the Real World and Dream World.
#2 Somnia Castle (Real World)
Items: Mini Medal & Platinum Shield
After entering the castle while still on the first floor go to the northeastern most part of the area to find a stairwell down. In this room you'll find a locked door as well as two treasure chests (pictured above). This location has treasure chests in both the Real World and Dream World.
#3 Port Haven (Real World)
Items: Mini Medal, 1500G
Go down the stairs in the southern portion of Port Haven, just west of Ella's house (we were here during the game's story). This is the Port Haven prison and you'll find a Locked Door that leads to two treasure chests (pictured above).
#4 Arkbolt Castle (Real World)
Items: Mini Medal, Optional Character Lizzie
Go up to the throne room of Arkbolt Castle and from here go into the staircase in the southeastern most portion of the area. On your way there you'll pass by the same location we go to for the Magic Key Locked Door in Arkbolt. In this location you'll find a pot containing a Mini Medal and the optional character, Lizzie, which you can only recruit to your party once Terry has joined you.
#5 Howcastle (Real World)
Items: Mini Medal & Platinum Mail
You'll find this locked door in the northeast (upper right) hand side of the first floor in Howcastle. It's right next to the Inn and extremely hard to miss.
#6 Poseidon's Palace (Real World)
Items: Kerplunk Bracer, Silver Platter, Mini Medal & Watermaul Wand
Just to the east (right) of where Poseidon is on his throne you'll find the treasure room (pictured above). You'll pass right by this room after coming down the stairs on your way to Poseidon. Don't forget to heal up after you collect everything since the treasures are surrounded by damaging floors.
#7 Mobile Island (Dream World)
Items: Mini Medal, Hela's Armour & Magic Water
While you are sailing around on the Mobile Island press 'A' to gain control of your character on the island itself. Go downstairs next to the bar and you'll find a treasure room with 3 chests for you to collect.
#8 Underwater Palace (Real World)
Items: Gringham Whip, Mini Medal, Tempest Shield & Mini Medal
You can access this place by using Lorelei's Harp while you're in your boat and sailing to the Underwater Palace at the northern end of the World Map (pictured above). It's found due east of Scrimsley and pretty easy to find. Each of the treasures in this dungeon are guarded by two guards which will be difficult to kill for anyone under level 35. If you have good equipment and a good strategy you can take them out as early as level 30.
#9 Felonia - Castle Jail (Real World)
Items: Mini Medal & Urnexpected
Inside of the Felonia Castle go down the stairs on the first floor to find the jail area (pictured above). You can use the Ultimate Key inside of here to open the two jail cells and get a Mini Medal and fight an Urnexpected or Frighturn which has a small chance of dropping another Mini Medal or a Seed of Resilience.
#10 Felonia - Doors Inside of Well (Real World)
Items: Seed of Agility, Mini Medal, Prayer Ring
Defeat the Scarecrow that guards the well and go down it. Inside you'll find a locked door that you can open with the Ultimate Key that leads to the room pictured above.
#11 Jail Island Northwest of Sorceria (Dream World)
Items: Magma Staff
This Ultimate Key Location can be found almost directly south of Slimopolis. It's on a small island with a forest and one or three tiny hills; you can land your Flying Bed on the northern tip of this island. You'll find a few items inside that you can obtain without the Ultimate Key and a jail cell with a treasure chest inside of it that can only be opened with the Ultimate Key.
#12 Castle Graceskull (Dream World)
Items: Mini Medal, Tempest Shield, Strength Seed
Reaching this Ultimate Key door is a bit tricky. You'll visit Castle Graceskull during the story quests of the game and you'll even pass right by these doors, however you'll do so in the Real World not in the Dream World. In order to access this door you'll need to return to Castle Graceskull in the Dream World (take the well from the destroyed Castle Graceskull in the Real World to reach this one like we did during the story).
Once you're in the Dream World version of Castle Graceskull you'll want to uncover the secret staircase that we uncovered in the Real World. For the exact location check out Chapter 9 of my Walkthrough. It's in the training room and you will need to use the Golden Pickaxe to make the stairs appear.
The hidden staircase down to Ultimate Key Doors.