Have you seen well in any other shape without round ?
Wells can be dug in any shape but mostly they are in round shape. Round wells are the strongest than any other shape of wells. There is no corner on the round shaped well because of which water exerts pressure on every side equal. If well is in square shape, then the water pressure will get apply on the corners rather than on the sides of wall.
It is easy to dug round well and can be build with the small quantity of materials. Round wells provide the additional strength from outside especially during the times of wind and storm.