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.