'The science of the great city; the physiology of London; literally and metaphysically the greatest subject that the mind of man can conceive. What an admirable _salmi_ this is; undoubtedly the final end of the pheasant. Yet I feel sometimes positively overwhelmed with the thought of the vastness and complexity of London. Paris a man may get to understand thoroughly with a reasonable amount of study; but London is always a mystery. In Paris you may say: 'Here live the actresses, here the Bohemians, and the _Ratés_'; but it is different in London. You may point out a street, correctly enough, as the abode of washerwomen; but, in that second floor, a man may be studying Chaldee roots, and in the garret over the way a forgotten artist is dying by inches.'
Again and again, as one more city or village or urban site is presented for role playing purposes, the vertical element is almost always forgotten or completely overlooked. Most cities, even medieval ones, had more than one level above ground. It's not just what's in the basement or sub-basement or pseudo-Parisian-esque sewers that we need to consider, but also what is going on up on that second floor above the ground floor, and whether there are any third floors or fourth floors. What goes on in those shuttered rooms, who lives in those vermin-infested garrets and what strange things lie forgotten or sealed away within all those dusty attics? There is serious potential for plots, complications, unexpected encounters, and even whole new scenarios and adventures that take place within crawlspaces, attics, balconies and other such elevated spaces. Think of a dungeoncrawl -- an Attic-crawl -- that takes place and across several floors above the street-level shops and homes.
Developing your urban areas along the lines of Neighborhoods and Streets is a great start, Judges Guild did this with The City State of the Invincible Overlord. They set up unique neighborhoods and streets where special encounters could be had based on where you went within the city. Each street had its own unique encounters and thus a unique personality unto itself. The infamous chamber-pot encounter is another one of those bits of local 'color', but like punctuation or punchlines, you don't want to overdo it. Judges Guild did a great job in establishing this approach. It worked then, it works now.
So after setting up a bunch of tables for basic encounters for the major streets, alleys, squares, etc. you might want to consider expanding those encounter tables to encompass a few of the various levels above ground that are available. There's something going on up on those roof-tops. How many times has a thief, spy or assassin used the roofs to gain access to a victim's abode or to escape from a job? Ever see Hitchcock's classic To Catch a Thief or the famous rooftop fight scene from Dario Argento's Cat O' Nine Tails or what about Crouching Tiger, Hidden Dragon? Rooftops are cool. They are at least worth including as a sub-type of urban terrain, with appropriate encounter tables and maybe some specials like weak spots, construction, broken or loose tiles, sneaky thieves, lurking assassins, snipers, or whatever.
Roof-tops are cool. Go exploring above the street-level for a change...