Go back to the locked door before 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 Elden Ring Runes you making sure to jump across the gap instead of falling down. Make sure to kill the remaining enemy within the middle of the platform. Then, carefully cross the wooden plank so that you can reach another enemy and a Fire Grease.
Take note of the slightly elevated wooden platform just to the left in the middle of the board. You can run and jump over it to access a door to the exterior. There you will meet the shielded soldier. Make sure you stand your ground and try not to get knocked from the platform. Continue to walk down the stairs and go through the door to your left. Here you will meet another soldier, as well as The Brick Hammer. Don't descend from the point at which you are, instead make a turn and walk back to the room with the ladder.
It is recommended to take the left side, but beware of enemies who are ready to attack just behind the stairs in front of you. Eliminate the enemy before climbing the stairs, only to Elden Ring Items for sale encounter another enemy resting at the top of the stairs, as well as a Silver-Pickled fowl Foot hiding behind boxes. Take a walk out the door and you will find an outside platform on which you'll find a huge staircase. Don't ascend it just yet instead, you should walk along the wall's edge to find Mushroom three times around the corner.