At the very southern tip of Roda, inside the Monastirli Palace compound, the Nilometer was constructed in AD 861. Like others built millennia before, it measured the rise and fall of the river, and thus predicted the fortunes of the annual harvest. The Turkish-style pencil-point dome is a Farouk-era reconstruction of an earlier one wrecked by Napoleon’s troops.

If the water rose to 16 cubits (a cubit is about the length of a forearm) the harvest was likely to be good, inspiring one of the greatest celebrations of the medieval era. Any higher, though, and the flooding could be disastrous, while lower levels presaged hunger. The measuring device, a graduated column, sits below the level of the Nile at the bottom of a flight of precipitous steps, which the guard will cheerfully let you descend for a little baksheesh.