Here is a comprehensive list of all Simple Key locations in Hollow Knight (main game) and their key usage locations:
Simple Key Locations in Hollow Knight (main game)
- City of Tears
- In a room to the top right, just below the City Storerooms stag station.
- Ancient Basin
- At the bottom of the loop west of the bench in the area with Mawlurks. Inside the large Mawlurk corpse, there's a dead Royal Retainer holding a Simple Key.
- Colosseum of Fools
- Behind a breakable wall to the right of the cage wall outside the arena. Defeat the Pale Lurker inside the secret room to get the key.
- Purchased from Sly in Dirtmouth
- Available for 950 Geo from shopkeeper Sly.
Key Usage Locations (Where Simple Keys Unlock Doors)
- Entrance of the Royal Waterways from City of Tears (West), in the room to the left of the fountain statue.
- Entrance to a cave right of Dirtmouth, past the graveyard.
- Entrance to the Pleasure Room in the City of Tears (East).
- Free Godseeker from her bonds inside the Junk Pit.
- Unlock Confessor Jiji’s home at the far right of Dirtmouth to exchange Rancid Eggs for calling your Shade.
These keys unlock doors with simple locks, and each key breaks after use, so they are consumable items. If information on the newer Silksong game keys is also needed, let me know! The above data is for the original Hollow Knight game.