There is a thieves' guild sewer entrance in the poor quarter. Talk to Lain (east edge, centre house); you have to join the thieves' guild first to get access. In the sewer, the entrance is to the far west, in the NW corner of the main section of the sewer. I think there is a ladder (or something) in the shadows where it would be hard to spot without knowing it is there, but the entrance must be opened from the poor quarter first.
