The brass key grants access to the back door of the Edgeville Dungeon. This is extremely useful as it shortens the distance between the ore in the mine and the bank in the Grand Exchange. Access to the back door is located in a house west of the Cooks' Guild. It is used mostly to kill the monsters that dwell within. These include hobgoblins, hill giants, zombies, skeletons, and giant rats. The dungeon is favoured by free-to-play players looking for a place to train; the hill giants provide excellent combat experience, especially to mages using air spells.
The key can be picked up inside the dungeon near the back entrance in a dead-end room guarded by level 29 zombies just north of the hill giants. This could be used as a moneymaking method: staying in the dungeon and collecting the keys, but the zombies can be aggressive and it is advised to take armour, a weapon, and food if you are a lower level player. The key respawns about 50 seconds after it has been picked up.
This key used to be used as a scam to trick players by saying it unlocks the gate to the store under Varrock west bank.