It's not interesting, just a drainage pipe beneath the bridge. Might lead somewhere important, but it's so out of the way it seems low priority to blockade.

[so Ydris is up here pretending the bridge is the true priority, not the passage beneath it. It's rather effective, people are focused on getting across the river or what is coming through it by boat, not so much the sewer channels beneath them]

Wouldn't advise going down there yourself, if only to avoid for the foul smell that will cling to your everything.