Rome was once the biggest city on the planet. What happened to everyone?

Rome may have not been built in a day, but it sure as heck fell pretty quick. It was once a shining example of a civilization, a bustling capitol that topped out around 1,650,000 citizens by 100 AD, until late in the 20th century where it reached a whopping 3,500,00 people. However, by the 14th century it fell to a paltry 17,000.

Between 1 and 100 AD Rome was the largest city in the world with the Roman Empire completely dominating the Mediterranean when the Republic ended. Augustus took over Caesar's grand projects and created many of his own including the Forum of Augustus and the Ara Pacis. In 64 AD the Great Fire of Rome destroyed most the city, but Rome used it as an opportunity for new development.

Thanks to a plague that killed around 2,000 people a day and numerous sackings of the city the Western Roman Empire went into many great declines. The declines accelerated when the capture of Africa Proconsularis by the Vandals took place in the fifth century. The grain supply from Africa ceased and the citizens fled, destroying monuments in the process.

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