On the Grandeur of Rome, is a collation of NH III.v.66-67, NH
  XXXVI.xxiv.101-110, NH XXXVI.xxiv.121-123. The direct links to the Latin text of this
  section of Pliny are possible due to the advice of Bill Thayer at the University of
  Kansas.
  [Davis Introduction]
  The following short sketch of Rome, its streets, buildings, etc., is given us by a
  careful author, writing in the reign of Vespasian (69-79 A.D.). While the area of Rome was
  far inferior to various great modern capitals, probably the masses of the population were
  so compactly housed that the inhabitants in Pliny's time numbered well up to 1,500,000,
  although any estimates must be very uncertain.
	
  [From Natural
  History III.v.66-67]
  Romulus left the city of Rome, if we are to believe those who state the very greatest
  number, with only three gates, and no more. When the Vespasians' were Emperors and Censors
  in the year of the building of the city, 826 [73 CE], the circumference of the walls which
  surrounded it was thirteen and two-fifths miles. Surrounding as it does the Seven Hills,
  the city is divided into fourteen districts, with 265 crossroads under the guardianship of
  the Lares [i.e., a little shrine to the Lares would stand at each crossing].
  If a straight line is drawn from the mile column placed at the entrance of the Forum to
  each of the gates, which are at present thirty-seven in number---taking care to count only
  once the twelve double gates, and to omit the seven old ones, which no longer exist---the
  total result will be a straight line of twenty miles and 765 paces. But if we draw a
  straight line from the same mile column to the very last of the houses, including therein
  the Praetorian camp [in the suburbs] and follow throughout the line of the streets, the
  result will be something over seventy miles. Add to these calculations the height of the
  houses, and then a person may form a fair idea of this city, and surely he must confess
  that no other place in the world can vie with it in size.
  On the eastern side it is bounded by the mound (agger) of Tarquinius
  Superbus---a work of surpassing grandeur; for he raised it so high as to be on a level
  with the walls on the side on which the city lay most exposed to attack from the
  neighboring plains. On all the other sides it has been fortified either with lofty walls,
  or steep and precipitous hills; yet it has come to pass, that the buildings of
  Rome---increasing and extending beyond all bounds---have now united many outlying towns to
  it. 
  
  [from Natural
  History XXXVI.xxiv.101-110]
  (adapted and excerpted)
  In great buildings as well as in other things the rest of the world has been outdone by
  us Romans. If, indeed, all the buildings in our City are considered in the aggregate, and
  supposing them---so to say---all thrown together in one vast mass, the united grandeur of
  them would lead one to imagine that we were describing another world, accumulated in a
  single spot.
  Not to mention among our great works the Circus Maximus, that was built by the Dictator
  Caesar---one stadium broad and three in length---and occupying with the adjacent buildings
  no less than four iugera [about 2 acres] with room for no less than 160,000 spectators
  seated---am I not, however, to include in the number of our magnificent structures the
  Basilica of Paulus with its admirable Phrygian columns [built also in Julius Caesar's
  day], the Forum of the late Emperor Augustus, the Temple of Peace erected by the Emperor
  Vespasian Augustus---some of the finest work the world has ever seen? [and many others].
  We behold with admiration pyramids that were built by kings, while the very ground
  alone that was purchased by the Dictator Caesar, for the construction of his Forum, cost
  100,000,000 sesterces. If, too, an enormous expenditure has its attractions for any one
  whose mind is influenced by money matters, be it known that the house in which Clodius
  [Cicero's enemy] dwelt was purchased by him at a price of 14,800,000 sesterces---a thing
  which I for my part look upon as no less astonishing than the monstrous follies that have
  been displayed by kings.
  Frequently praise is given to the great sewer system of Rome. There are seven
  "rivers" made to flow, by artificial channels, beneath the city. Rushing onward
  like so many impetuous torrents, they are compelled to carry off and sweep away all the
  sewerage; and swollen as they are by the vast accession of the rain water, they
  reverberate against the sides and bottoms of their channels. Occasionally too the Tiber,
  overflowing, is thrown backward in its course, and discharges itself by these outlets.
  Obstinate is the struggle that ensues between the meeting tides, but so firm and solid is
  the masonry that it is able to offer an effectual resistance. Enormous as are the
  accumulations that are carried along above, the work of the channels never gives way.
  Houses falling spontaneously to ruins, or leveled with the ground by conflagrations are
  continually battering against them; now and then the ground is shaken by earthquakes, and
  yet---built as they were in the days of Tarquinius Priscus, seven hundred years
  ago---these constructions have survived, all but unharmed. 
  Passing to the dwellings of the city, in the consulship of Lepidus and Catulus [78
  B.C.] we learn on good authority there was not in all Rome a finer house than that
  belonging to Lepidus himself, but yet---by Hercules!---within twenty-five years the very
  same house did not hold the hundredth rank simply in the City! Let anybody calculate---if
  he please---considering this fact, the vast masses of marble, the productions of painters,
  the regal treasures that must have been expended in bringing these hundred mansions to vie
  with one that in its day had been the most sumptuous and celebrated in all the City; and
  then let him reflect that, since then and down to the present, these houses had all of
  them been surpassed by others without number. There can be no doubt that the great fires
  are a punishment inflicted upon us for our luxury; but such are our habits, that in spite
  of such warnings, we cannot be made to understand that there are things in existence more
  perishable than even man himself.
  
  [from Natural
  History XXXVI.xxiv.121-123]
  But let us now turn our attention to some marvels that, if justly appreciated, may be
  pronounced to remain unsurpassed. Quintus Marcius Rex [praetor in 144 B.C.] upon being
  commanded by the Senate to repair the Appian Aqueduct and that of the Anio, constructed
  during his praetorship a new aqueduct that bore his name, and was brought hither by a
  channel pierced through the very sides of mountains. Agrippa, during his aedileship,
  united the Marcian and the Virgin Aqueducts and repaired and strengthened the channels of
  others. He also formed 700 wells, in addition to 500 fountains, and 130 reservoirs, many
  of them magnificently adorned. Upon these works too he erected 300 statues of marble or
  bronze, and 400 marble columns, and all this in the space of a single year! In the work
  which he has written in commemoration of his aedileship, he also informs us that public
  games were celebrated for the space of fifty-seven days and 170 gratuitous bathing places
  were opened to the public. The number of these at Rome has vastly increased since his
  time.
  The preceding aqueducts, however, have all been surpassed by the costly work which has
  more recently been completed by the Emperors Gaius [Caligula] and Claudius. Under these
  princes the Curtian and the Caerulean Waters with the "New Anio" were brought a
  distance of forty miles, and at so high a level that all the hills---whereon Rome is
  built---were supplied with water. The sum expended on these works was 350,000,000
  sesterces. If we take into account the abundant supply of water to the public, for baths,
  ponds, canals, household purposes, gardens, places in the suburbs and country houses, and
  then reflect upon the distances that are traversed from the sources on the hills, the
  arches that have been constructed, the mountains pierced, the valleys leveled, we must
  perforce admit that there is nothing more worthy of our admiration throughout the whole
  universe.