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.

Ground plan and cross section of the cistern that supplied water to the St. George Barracks (F. Basilicata)
Plan of the town aqueduct and St. George cistern, 1900 - 1905 (G. Gerola, Vikelaia Municipal Library, Heraklion)