The keys are: the Simple Key, of which there are three in the game and one is sold by Sly for 950 Geo, used on the Jailer's Cell in the Ancient Basin, the Junk Pit door in the Royal Waterways and a door in Deepnest. The Elegant Key, sold by Sly for 800 Geo, opens the Watcher's Spire in the City of Tears. The Love Key, found in the Queen's Gardens, opens the Tower of Love. The Shopkeeper's Key, found in the Pleasure House, opens Sly's upstairs room.
The other essential items are the City Crest, dropped by the Soul Warrior and needed to open the City of Tears gate, the Tram Pass from Deepnest for the tram to the Ancient Basin, the King's Brand from Kingdom's Edge for the White Palace, and the Lumafly Lantern from Sly for 1800 Geo, which lights the dark areas.
How to collect them
Buy the Lumafly Lantern first. Deepnest and the Abyss are both effectively closed without it, and both hold items you need. After that, the Elegant Key at 800 Geo is the best value because it leads to a Dreamer. The Simple Keys are the awkward ones: there are only three and four possible doors, so you have to choose, or find all three.
Filter the map here to the Keys and Items category to see which doors you have left and what opens them. If you are unsure which Simple Key door to use first, the Junk Pit in the Royal Waterways contains another Simple Key, so opening that one costs you nothing net.
What you get
Key items give no completion percent by themselves, with one exception: the King's Brand counts as one of the seven equipment items and is worth 2 percent. Everything else is access. The Elegant Key leads to Lurien, the Love Key leads to the Collector and three grubs, the Lumafly Lantern opens two whole regions, and the City Crest opens the largest area in the game.
All 24 locations by area
Dirtmouth (6)
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Sly's shop
The only Simple Key you can buy. Sly never restocks it, so spend it carefully.
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Sly's upstairs storeroom
On sale once Sly has the Shopkeeper's Key. It opens Emilitia's door in the Pleasure House.
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Sly's shop
The lantern lights the pitch-black rooms of Deepnest and the Crystal Peak mines. Buy it before heading down.
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Inside the Grimm Troupe tent
Defeat Grimm in the troupe tent in Dirtmouth and the notch is yours.
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Iselda's map shop
Buy the Quill from Iselda so the map fills in whenever you rest on a bench.
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Iselda's map shop
A charm sold by Iselda that shows your position on the map. It goes on sale once you have met Cornifer.
Forgotten Crossroads (5)
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False Knight arena
Dropped by the False Knight. Slot it into the statue in the Fungal Wastes to open the gate into the City of Tears.
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Salubra's shop, east Crossroads
The first notch Salubra offers. She only sells it once you own 5 charms.
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Salubra's shop, east Crossroads
Unlocked in her stock once you own 10 charms.
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Salubra's shop, east Crossroads
Appears once you own 18 charms.
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Salubra's shop, east Crossroads
Her last notch, on sale once you own 25 charms.
Fungal Wastes (1)
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Shrumal Ogre arena, north Fungal Wastes
Defeat the two Shrumal Ogres in the north part of the Fungal Wastes and the notch is left behind in their room.
City of Tears (2)
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South of the City Storerooms
In the topmost alcove on the right side of the room south of the Storerooms, in the north-west City.
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Top of the Tower of Love
Found at the top of the Tower of Love after defeating The Collector. It pins every unfreed Grub on your purchased maps.
Crystal Peak (1)
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Room left of the Crystal Guardian bench
In the room to the left of the bench room where the Crystal Guardian is first met. Give it to Sly to unlock his storeroom.
Resting Grounds (1)
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Grey Mourner's house
The Grey Mourner gives you the flower for the grave in the Queen's Gardens. It breaks if you take a hit, use a bench or fast travel.
Fog Canyon (1)
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Hidden area north-east of Cornifer
Float or swim up into the sealed pocket north-east of where Cornifer sits in the fog. Either Isma's Tear or the Monarch Wings will get you in.
Royal Waterways (1)
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Junk Pit
Open the Godseeker's cocoon in the Junk Pit with a Simple Key. She drops the Godtuner, which unlocks Godhome and the Pantheons.
Deepnest (1)
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Inside the wrecked tram, Failed Tramway
Climb into the broken tram car at the Failed Tramway. The pass turns on both working tram lines.
Ancient Basin (1)
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South-west Ancient Basin
Held by a Royal Retainer corpse that a Mawlurk has swallowed. Kill the Mawlurk to reach it.
Kingdom's Edge (2)
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Hidden room behind the Colosseum of Fools
Added with Godmaster. Beat the Pale Lurker in the junk room behind the Colosseum to make her drop the key.
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Cast-Off Shell, far east Kingdom's Edge
Guarded by Hornet at the Cast-Off Shell. Beat her, then take the brand from the very end of the shell. Normally needs the Monarch Wings.
Queen's Gardens (1)
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South-east Queen's Gardens
On a large Husk Dandy corpse above the path back to the Fungal Wastes, guarded by a pair of Mantis Traitors. Isma's Tear is needed to retrieve it. The key unlocks the Tower of Love.
Colosseum of Fools (1)
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Trial of the Warrior reward
Complete the first Colosseum trial and collect the notch afterwards.