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	089 of 152. The architect-engineer Antonio da Sangallo the Younger surrounded the central well shaft with two spiral ramps in a double helix, accessed by two doors, which allowed mules to carry empty and full water vessels separately in downward and upward directions without obstruction. The cylindrical well is 174.4 ft deep with a base diameter of 43 ft. There are 248 steps and 70 windows provide illumination.