Return to the stairs and proceed up to Elden Ring Items the platform at the top. The enemies will throw firebombs at you in order to light the barrels at the top of the staircase. Roll backwards until the explosion starts after which you can run up the stairs and get rid of the guards before they can bombard you with more bombs. On this platform, you'll find a locked gate that we can't access right now, but go just to the right of door for the Throwing Dagger x5.
Then turn around and head up the next step, and you are in a room with a Smoldering Butterfly x3 and more explosive barrels for you to avoid. Kill the enemy aheadof you, then smash the debris in the back and you will be able to see another set stairs. Kill the enemy at high up on the stairs, then lower to grab the Golden Rune  on the platform below you.
You can go back up the stairs and open the black door to buy Elden Ring Runes the right of the upper platform. If you open the door, it will trigger a cutscene where the door is shut behind you, sealing you in with an imposing black knight. The knight has a big shield, however his slow moves make him very vulnerable to backstabs. Be aware, however. There's a great chance that the knight will use its Ashes of War Storm Stop ability to summon an energy gust that can cause serious damage, even if you're wearing a high-end armored character.Once the knight has passed away, you can explore the room and grab and take the Rusty Key and Curved Sword Talisman.