This structure is called the Nilometer, the level of the Nile | World Theatre
This structure is called the Nilometer, the level of the Nile River was measured. This nilometer is located on the island of Roda in Cairo.
To measure the depth of the river, it is enough to lower the rope / stick in the right place. But no, you need to build a whole "castle" to find out the depth of the river. There are many such buildings and all for measuring the depth
I am sure that the original purpose of these structures was not clear even to the Egyptians themselves.
