I'll start off by saying I thought this chapter was easier to follow than the last (one of the drawbacks of jumping into the middle).
Right off the bat I got various details that intrigued me. The first was a watchmaker/repairman being one of the main characters, partially because it places the time period a little later than I had previously assumed. The next was that the Mother's body talked to Lorn. I'm sure this event was covered previously, but it made me take closer notice.
Dialects in text can be hard. For me, Darl's was a struggle to read with all the dropped 'G's in his sentences. This mostly struck me at the beginning of the chapter. I think you did less of it later on. On the other hand, I thought Fit's occasional use of "me" instead of "I" was well done. (I think it was Fit speaking those times.)
Speaking of those two, there were a number of places where I got confused as to which of them was speaking. This was usually because you would have one speak, describe the motions of both or just the other, then have the first speak. Alternately, I think once you described the motions of the first, then had the second speak. In each case, both dialogue texts would be in the same paragraph.
Along similar lines, there were a few places I thought text could be rearranged or cut for better effect. Almost all of these came as actions interspersed with the dialog, to the point that, for me, the dialog was slowed down too much and didn't flow well. If you would cut what you can and group the actions and thoughts together more where you can, I think it would be improved. (One example is near the beginning, where Lorn thinks the two are loyal, but not thinkers, says what the Mother must have done, then thinks again about the two of them not surviving long without him. If instead the parts about the two others were together, then the revelation, it would make more sense to me.)
Fit saying "we can't go through the gate", phrased like that, made me think he was just repeating the obvious. Perhaps that's what you want, but I would guess you might want something more like "we can't even get through the gate".
Also in that section, Fit and Darl look down often.
How do they know they'll need a written map before they know how much detail there is to the sewers?
I liked the paragraph with watch details, but I personally thought there were a couple unnecessary details; for example, what happened to cause the watch to rust. To me, it's enough that it has rusted on the inside, but nobody will care how much gets changed as long as it looks the same.
At the end of that section, Lorn's introspection seemed a little off to me. I've started using the character responses I learned about from Jim Butcher's blog at
http://jimbutcher.livejournal.com/2880.html and I think it has helped me do better with them.
Darl and Fit return to a common room, which immediately makes me think of an inn, although I doubt they would be plotting (or Lorn repairing watches) in an inn. I could be wrong.
I wondered how common iron grates over sewer outlets were historically, and if a city would install them as mentioned. After all, if everything is falling a couple yards/meters/spans before hitting the sewer pipe anyway, why worry about fishes? Also, the word "conduit" is very modern, to me, which didn't help immediately after thinking about grates. Later on, I also wonder why there would be two grates on the end of the pipe.
Dialog tags. It was most obvious with Darl, who (besides said) mocked, answered (x2), laughed (x2), and replied (x6). I know the others did a lot of this as well.
Finally, Lorn climbs into the library and cuts his hands all up -- with sewer slime on them. To me, this seems like an instant recipe for infections and illness. I hope something goes wrong because of it.
