The Fortress of Babylon is the Roman military installation that underlies Coptic Cairo — built by the Emperor Trajan (1st-2nd century AD) to control the Nile crossing and expanded repeatedly into the 4th century. The massive round towers of the original fortress are still visible today, incorporated into the structure of the Coptic quarter: the Hanging Church sits on top of the south gate, and the round towers of the Water Gate are visible in the archaeological area near the Coptic Museum.
The remains are best seen at the Water Gate archaeological area, where excavations in the 1990s exposed the original Roman masonry and the water level from when the Nile ran directly alongside the fortress. The scale of the original structure — with walls three metres thick and towers 30 metres in diameter — is impressive even in fragments.