France Deep Well

This is a shot down the central well of the Citadel in St.Tropez taken during a recent visit to the South of France. I do not understand how this structure which goes down perpendicularly for many metres can be built .Can someone out there explain the means? Do they start from the surface , if not how are the sides supported while the stone is put in place? It all seems impossible. Please email me if not room on comments to normanhaddow@hotmail.co.uk