You're looking at them, or a stylized version of them. With few exceptions the Roman urban population lived in insulae, tenement blocks that were packed close together--the wealthy, of course, could afford real houses. Real insulae would have been packed closely together, often sharing a single thin wall between two buildings. Though attempts were made in the Principate to introduce primitive building codes they were generally ineffective, except in smaller urban areas like Ostia, where we get rather nice insulae like this one. Insulae were often built as high as possible, to cram more people into a smaller area, and as cheaply as possible--frequently their walls were built of rubble concealed by a thin facing of brick or stone, and insulae collapsed with alarming frequency. And when I say cram I mean cram. Insulae were packed, with entire families living in one room without any amenities, and generally no windows either--at Ostia I took a picture of those same "nice" insulae from above to show the rooms. Those are all the remains of individual rooms, in which families lived stacked on top of each other and only sharing a thin brick wall between them. As I said, the insulae at Ostia are pretty nice, and even conform with later imperial building codes--I should add that they're even smaller than the picture makes it seem, they're absolutely tiny. Besides collapsing insulae caught fire with alarming regularity, and fires spread between them quickly--the shoddy building materials and narrow alleyways of Rome made the blocks of the city particularly prone to lighting up (the jurists actually have a comment on the narrowness of the streets in the city, debating whether yo can sure if a building next to yours catches fire and its overhanging roof--a common feature in these choked alleys--sets yours on fire). But technically speaking those are insulae in those artist's representations, or at least a lot of them are. They're not as high or as dense as real insulae would have been, but they're recognizably insulae, generally reconstructed from the surviving examples in places like Ostia--make no mistake, though, the seemingly durable construction of such buildings was generally a sham, and the interior rarely reflected the exterior. It should also be added that for many of the urban poor there was no permanent place to live. The vast majority of urban inhabitants were unskilled day workers who roamed the streets searching daily for work. Rent in the city was frequently paid on a daily basis, and as workers and their families migrated through the city looking for someone to hire them they might find themselves living somewhere very different from day-to-day, or perhaps temporarily homeless--Rome probably had quite a large population of itinerant, semi-homeless workers migrating through the city