Just who Cestius was is not really known. Cicero mentions a Praetor named Cestius in one of his books, but it is not certain that this tomb belonged to the same Cestius. Like almost all ancient Roman tombs, this one was outside the existing city walls, those supposedly built by Rome's sixth King, Servius Tullius, when it was dedicated in 12 AD. When the Emperor Aurelian walled the much expanded city starting in 271 AD, Cestius' pyramid became part of the new fortifications, close to the Porta Ostiensis (Ostia Gate), now known as the Porto S. Paolo.
The core of Cestius' pyramid is good brick-faced Roman concrete on a solid travertine base. Its outer surface is overlaid with slabs of white Italian luna marble. It is 36 meters high and 30 meters square, much smaller than the great Egyptian pyramids (Khufu is 147 meters high and 230 meters square). The burial chamber inside is about six meters by four meters and five meters high with painted and stucco decorations that are of a style inconsistent with the age of the tomb as determined by its inscriptions -- the tomb must have been redecorated and perhaps was reused in the second or third century AD. Drawings of the interior decorations were made in the late 17th century, but they are now badly faded. Neither Cestius nor any possible reusers of his crypt are at home -- the tomb was robbed long before excavations and renovations took place during the reign of Pope Alexander VII in the 1660s.
The side of the pyramid that almost everyone has seen is the one outside the Aurelian walls, but the northwest face, visible from the Protestant Cemetery inside the Aurelian wall, is more imposing and more serene because it is protected from automobile traffic and because it also is not burdened by an ugly modern sign. Both the inner and outer walls bear inscriptions naming Cestius as the owner and giving his various titles and offices. Other inscriptions tell of the construction of the pyramid and of the Alexandrine cleaning renovations. Two pillars were re-erected and the bases for two bronze statues were found on the inner side, but only small remnants of the statues, one arm and one foot, were found.
The Pyramid of Cestius was again cleaned in 1999 in preparation for year 2000 millennium celebrations.
There are good Internet links, but they really are only a shadow of the real thing, which is easy to get to and always available for viewing. Internet links:
Lacus Curtius Gazetteer links: http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/Pyramid_of_Cestius/links*.html
Platner and Ashby Topographical Dictionary entry (with a very good picture from the Protestant Cemetery side, but before the 1999 cleaning): http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Sepulchrum_C.Cestii.html
Porto S. Paolo and Pyramid Images from Koskimies: http://www.siba.fi/~kkoskim/rooma/pages/APPAOLO.HTM
Piranesi's evocative drawing: http://www.paceprints.com/old/piranesi/images/piranesi-thepyramidofcaiuscestius_20-672.jpg