We find the cistern in a holy place, a sanctuary, a sanctified enclosure, a hieron, a Temenos.
Early Christian basilicas often had piscinas, fish ponds into which Christians were dipped; the idea was fishing believers out of the pond.
Early Christians wore a ring with two fishes on it and the net used in this fishing for believers figures on the Pope’s ring.
In this dream the cistern is not in the cathedral, but under it, in a secret hidden place.
This indicates that something in antiquity, under the Christian layer, is intended; we must dig through the layers before Christ in order to discover it, and it must if possible be connected with a snake.
We do come on such figures in antiquity.
There is a Neolithic Temple in Malta, built in an irregular shape into the rock, a precipitous place leads to such a hole with water in it, breaking through into a dark corridor.
The terra-cotta figure of a sleeping woman has been found in a niche: she is sleeping the incubation sleep, that is, sleeping for a definite purpose.
In Aesculapian Temples the sick were laid in the Temenos in order to have a healing dream, in which the gods handed them the right means for their cure.
In these antique temples the cistern was covered by a stone and sometimes a snake was set to watch over the Temple treasure.
The snake is the attribute of Aesculapius, the doctor, and was identified with him.
The Epidaurus snake was brought to Rome during the plaguebecause the Romans in their great need insisted on the presence of the God himself. ~Carl Jung, Modern Psychology, Page 185.