Back on track
Enter the tower again and go upstairs this time. Be careful of the undead prisoner here since it can deal huge amounts of damage when it slams the ground with its body. The door doesn't lead anywhere so ignore it. Pull the switch but be careful; expect three of those undead prisoners again to rush towards you. When you see them diving towards you, quickly roll out of the way to avoid getting damaged.
Continue along but make sure to roll when you enter the next room; an undead prisoner will leap to you from above. The concrete containers here contain more prisoners; they will break free as soon as you go upstairs. Going upstairs is a bad idea since there are more waiting there. The best way to handle this is to backtrack to the previous hallway where you came from so you can deal with them one or two at a time.
After clearing them all, continue to the second floor and pull the switch to lift the gate leading to a misty doorway. Open the door under the stairs to get a Skeptic's Spice. The corpse on the ledge below has a Bonfire Ascetic; however, you won't have any way of climbing back. The ledge below leads to the rooms that can be revealed by a Pharros Contraption, which we already covered earlier. It's a pretty long drop and even with the Silvercat Ring equipped, you won't survive it unless you have a lot of HP. If you don't want to risk it, use a Homeward Bone to return to the last bonfire, in exchange of enemy respawns.
In the same floor where you hit the switch leading to the pathway to Sinner's Rise, don't go up the third floor carelessly since there are a lot of those undead prisoners waiting. There's a bonfire there but a statue is blocking the way in. Ignore it for now go through the gate.
Before moving on, check your immediate right to find a royal guard guarding a corpse. Defeat it and loot the corpse for x8 Lifegems. Run across the walkway and avoid the bolts from the crossbowmen. Enter the white mist to reach a new area.