I have tried searching for this--it is a mod I got back when Oblivion was Beth's most recent game, but it's hard to describe it in a way that search engines get. Please bear with me (sorry for the length of this post).
I don't know if it was classified as a city or something else (I doubt it was a home, maybe a vendor mod, but it was much closer to being a small city). Somewhere in the part of the Niben which circles the capital, one would find an unusual ship/barge. It looked--on the outside--like an NPC patched together bits of old scows, so it had a very 'poor-ish' look. It wasn't an open at all IIRC--just jumped on and entered it.
The interior was the brilliant thing about this mod. In exploring the 'inside' it became clear that whomever (again, in in-world terms) had built it, had integrated the barge/ship with underground spaces; and the mod-maker had, in the most seamless and incredible way, incorporated different architectural styles (including Aylied). Walking around the interior make you think that someone who was a master of architectural design created this space. (Or you would think it was chaotic, or lore-breaking, etc.)
The overall 'vibe' of the place was very underworldly--very suited to thieves and the like. I know there was at least one merchant, but there may have been more. There was a bar area inside, I think at the uppermost area, but I recall there being lots of small areas throughout, which made it feel vary multipurpose.
Unfortunately, it's been so long since I have played Oblivion, so I no longer have the mods I once used, but I remember that for overall design this was one of my absolute favorites.
Thank you so much for whatever assistance can be provided.