Sewers is the shortest level of the game. There is a lot of dead-ends and several rooms with envelopes on the cupboards and in drawers. In this stage, Witches more often appear directly in front of you. Key to the magic door is at the end of one of the corridors. This time, root is a key.
When you find a root, run to the passage. Behind a passage you are in a safezone. Here, you have to lower the bridge.
First envelope is on the left side, next to the passage.
Three more envelopes are on the chests in room in front of the bridge.
Second room is at the end of the corridor. There you find a switch.
In this room you can notice three more remnants: on the chest behind a door, on a chair and on the books lying on the ground.
Now return to the mechanism of the bridge and activate it. Move to the other side.
At the end of the corridor you spot another envelope.
Now you can exit Sewers. Head to the small port on your right. Now you're again in the safezone, so you don't have to hurry.
When you get onto wooden stairs, you notice an envelope on the beam
On the next beam you find another secret. Turn around and approach the gasoline bottle.
Move closer to the knob and use it. Now go to the end of the hose from where gasoline runs.
Fire the flare and use it on the gasoline. If you don't have any flare, pick some from the chest attached to the pillar.
Go farther on the pier. On one of chests you find an attached note.
Approach the house. Between chests you notice another remnant.
Behind a window you see a chest with ropes. Move it closer to the remaining chests and climb onto the roof.
Inside the building you find a note attached to the wall
Another envelope is on the barrel, next to the door. After collecting them, exit the building.
Before you head for stairs, turn left and take a note from the chest.
Another one lies next to the fence. There are two left to be found.
Last but one envelope lies behind a chest next to the trash.
The last one envelope is at the end of the last pier. Then return and use stairs to go to the forest.