Opposite the Prefecture building, in the defensive ditch below the Venetian wall, was a large cistern supplying water to the St. George barracks.

Provveditor General Beneto Moro designated the site; the project was begun by General Capelo and completed by Francesco Morosini. 76.7m long and 24.5m wide, it was the most important cistern in Chandax; G. Gerola termed it a "sea". Water was supplied from the spring at Agia Irini via the Three Arches, which stood in what is now Eleftherias Square.