What was thought to be just a cistern holding a reservoir of water, turns out to be a shallow well! Cool! The well was dug by hand. The diameter is large enough to accommodate men with shovels digging down to below the water table. This was most likely a community project. The walls were lined with wet laid stone to prevent cave in and reduce surface water run in.
The depth of this well is 25 foot. The first water table reached is at a depth of 8 feet. The extra depth below the water table allows for additional storage. It is actually even possible that there is more than just one source of entry of water!