Despite its humble beginnings in 8th century BC as a series of small villages on the Seven Hills of Rome, by AD 100, the Roman Empire comprised up to 100 million people spread across 50 different modern-day countries.
Yet, following a series of crises from the later 2nd century AD, ancient Rome's decline began. The western empire fragmented rapidly before the final emperor was deposed in AD 476. However, the Eastern Roman Empire - later known as the Byzantine Empire - continued to thrive with its capital at Constantinople, surviving until AD 1453.