In Elden Ring, if you decide to enter Stormveil Castle via the cliffside path, and not via the main gate, then soon after entering the castle on the west side, you’ll find a locked door on the second floor. In order to open this door, you need to use the Rusty Key, which fortunately isn’t far away.
Go up the stairs opposite the locked door, and deal with the guard at the far end of the narrow passage (ranged attacks will make this much easier). Smash the boxes in the room at the end, then double back and go up some more stairs. Kill the enemy here, then open the door next to where he was sitting.
Related: How to complete Roderika’s quest in Elden Ring
There is a tough knight in the corner to the left as you enter the darkened room beyond. You can fight him if you want to, but you don’t have to in order to get the Rusty Key. The Rusty Key is on the corpse directly opposite the door you enter by, so you can just run across the room, pick up the key, and try to escape before the knight catches you. Even if the knight kills you, you’ll still keep the Rusty Key. Open the door with the Rusty Key to access a ladder leading to the upper floors of Stormveil Castle.
