This is an ancient pyramid, built around 18-12 BC as a tomb for Gaius Cestius, a magistrate.
It stands next to "Porta San Paolo" (San Paolo Gate), one of the gates of the Aurelian Walls that -still- surround the old part of Rome. It also stands at the beginning of Via Ostiense, the road that links Rome with Ostia -the main port in Roman times.
© Benito Jiménez-Sauma Photography
