Return to the platform using the locked door. Don't try to use the key just yet. Instead, look for the massive hole in this wall, to Elden Ring Runes the left behind the exit. Then, jump into the hole within the wall, where you will see The St. Trina's Arrow 10. Make sure to target the enemy below and hit them with the fall attack. Beware of a spear-wielding enemy on your left who will attack you with firebombs and a third one who will descend from the ladder at high on the ladder. Once all the enemies are removed, climb up the ladder and get back inside the room by using the hole in the wall near the top.
Return to the locked door from earlier, and unlock it with Rusty Key. Rusty Key, then climb the ladder to the platform on top. When you are out of the door, take an right turn towards the platform ahead of you, taking care to jump over the gap and not fall down. Make sure to kill the remaining enemy on the platform, then carefully traverse the wooden plank to reach another enemy and a Fire Grease.
Take note of the slightly elevated wooden platform located to the right from the wooden plank. It is a good idea to run and jump onto it to access a door towards the outside. Here you will encounter an unarmed soldier. Stand your ground and try not to be knocked down the steps. Follow the steps down and enter the door on your left. Here you will meet another soldier as well as elden ring weapons for sale that's the Brick Hammer. Don't descend from the point at which you are, instead you should turn around and walk all the way back to the room where the ladder is.