If you have some time to spare in the area, this is an interesting place to see. Dating from early Islamic times (though the roof, and beautifully decorated ceiling, is Ottoman), it measured the level of the Nile which allowed officials to judge when to open irrigation canals as well as how successful the harvest might be (which in turn affects the tax rate). The concept is an ancient one in Egypt but this one is historically important for Cairo.
Look for the keeper to purchase a ticket and open the door. He may offer to let you go down the steps into the well/shaft, though I don't know if this would require extra baksheesh.
The site is easily walkable in 10-15 min from Coptic Cairo: you just have to cross the train track and the neighborhood on the other side till you reach the banks of the Nile, from which there's pedestrian bridge to Roda Island.
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