It has a small cell and several connections to the sewers. It also has the controls to the alarm system of the Maritime Museum, which are being installed by a grobo. There is also a Quetch nurse.
Twinsen can get imprisoned there if he gets caught in the Maritime Museum. To escape, he must use a secret elevator in the cell below the air duct that leads to the sewers.
When Twinsen has to infiltrate the maritime museum to steal some objects, after having used the elevator to the sewers in the museum, he has to activate the alarm from the prison. Then he can return to the sewers and get into the museum from the same elevator.