It's pretty much Des in a nutshell
I'd say that'd be under the Old Town district. It's like those small parts of the cities that have been around the longest. Then again, we pretty much assume that the Underground is actually an earlier version of Mandalus.
[ See? They've done some kind of digging around. ] Nah, someone else. He's not here in Mandalus. [ Desmond doesn't even want to imagine Connor being in Mandalus. He'd live out in the woods. ] Ezio taught me plenty of things. These? [ He nods to the rope dart and tomahawk. ] We're talking 1700s. Revolutionary War America.