Nilometer - Egypt was ruled from here

Cairobserver  

The level of Nile water has been measured for 5000 years to predict farming production. This current Nilometer, or Miqias in Arabic, on the southern tip of Roda Island dates back to 715 AD making it one of Cairo’s oldest sites in relatively good shape but also it is the second oldest building by the Ummayyads (Arab/Muslim) dynasty.

The Nilometer is essentially a measuring device, one big enough to be entered to occupy its interior space. The device consists of a central column with measuring markings of cubits (roughly the size of a forearm) and smaller units which are roughly the width of a finger. Although the original structure was restored in 815 AD it was destroyed in 850 AD by a flood. This has resulted in the addition of several chambers for the water to enter through before making it to the measuring hall (this would slow down the force of the water so that it isn’t destructive). Another addition to the structure following its 850 destruction was a carved wooden beam that supports the top of the column which had been free standing. Also a column capital was added. The current incarnation of the Nilometer was designed by Abu’l ‘Abbas Ahmad ibn Muhammad ibn Kathir al-Farqhani from Farghana, known in the West as the astronomer Alfraganus.