Limited water was exploited with an extensive cistern and aqueduct system, wells, and sanitary drains. Some toilets also use the siphon principle to obtain the actual flush from the cistern. Lymphatic drainage is to the chyle cistern. The hydraulis has a reservoir of air which is inserted into a cistern of water. It is built over a 200,000 gallon cistern that collects filtered stormwater from the roof. The peristyle courtyard was centred upon a rectangular cistern. The tower was meant to be a hermitage, having neither electricity nor a flushing cistern. Rain water passed through a sand filter and collected in the cistern. The water was collected in a large cistern near the Agora. The mechanism consists of a plastic tank hidden inside the typical ceramic cistern. The Pool of Arches, an underground water cistern, is currently under restoration. His house is destroyed in the Blitz and replaced by an emergency water supply cistern. Water was supplied by a well or cistern. Located outside the walls, this was perhaps the spring that filled the citadel's underground cistern. The vat held a steadily increasing amount of water, supplied by a cistern.