A 4-tefach monument/wall that separates the balcony from the courtyard.

Who can make use of projections from a wall shared by a lower and upper balcony?

If a well is located half in each of two courtyards and runs underneath the separating wall there has to be a wall in the water itself if no eruv was made. The wall does need to extend more than four tefachim into the water (based on פי תקרה יורד וסותם).