The inside of the Water reservoir (castellum) at Pompeii.
The passage below is a description of how a Castellum should work, according to the Roman architect Vitruvius. It doesn't fit very well with what has been found in the archaeological record, however, and it has been suggested that this is a theoretic suggestion rather than a practical solution.
"When they [The water channels] are brought home to the walls of the city a reservoir (castellum) is built, with a triple cistern attached to it to receive the water. In the reservoir are three pipes of equal sizes, and so connected that when the water overflows at the extremities, it is discharged into the middle one,
2. in which are placed pipes for the supply of the fountains, in the second those for the supply of the baths, thus affording a yearly revenue to the people; in the third, those for the supply of private houses. This is to be so managed that the water for public use may never be deficient, for that cannot be diverted if the mains from the heads are rightly constructed.º I have made this division in order that the rent which is collected from private individuals who are supplied with water, may be applied by collectors to the maintenance of the aqueduct."
Vitruvius - On Architecture VIII. 6.1-2
Translated by Gwilt 1826