Heading for Sinner's Rise:

As soon as you enter the tower, head upstairs. You'll run into a prisoner, similar to ones you might have seen before if you interacted with that stone near the well. Don't bother with the door you see here; it doesn't lead anywhere important.

You'll need to pull a switch, but be ready. Three more of those undead prisoners are going to rush you. When you see them charging in, your best bet is to quickly roll out of the way to avoid taking damage.

Before you enter the next room, be on your guard. An undead prisoner is going to leap down at you from above. Dodge out of the way, take care of him, and then you can proceed.

Inside this area, you'll notice some concrete containers. More prisoners are hidden in these and will break free as soon as you head upstairs. Going straight upstairs is a bad idea because there are even more waiting for you. The easiest way to handle this is to backtrack to the hallway and lure them out one by one so you can pick them off safely.

Once that's clear, head upstairs again and pull the switch to open the gate. You'll then come across a misty doorway.

Look for a door under the stairs – opening it will get you a Skeptic's Spice. There's also a corpse on a ledge below that holds a Bonfire Ascetic, but here's the catch: you won't be able to climb back up from there. That ledge actually leads to some rooms that are only accessible if you use a Pharros Contraption, which you might have encountered earlier. It's a really long drop, and even with the Silvercat Ring, you'd need a ton of HP to survive it, which you probably don't have at this point in the game. So, your best bet is to use a Homeward Bone to return to the last bonfire you rested at.

Head back to that misty door. Just a heads-up, there are more undead prisoners waiting for you on the third floor. There's a bonfire up there too, but a statue is currently blocking the entrance. For now, just leave it be and come back later.

To your right, you'll find a royal guard. He's protecting a corpse that has eight Lifegems on it. Take out the guard and grab the loot.

Now, you'll need to sprint through the walkway. Keep moving to avoid the arrows being shot by the crossbowmen. Your goal is to enter the mist at the end, which will finally lead you into Sinner's Rise.