This huge cistern dating from the Byzantine age (6th-9th centuries AD.), with a capacity of about 800 m3, was built when a settlement was built on the Rock to defend the local population from the frequent Saracen raids. The problem of the absence of drinking spring water on the Rock was solved by creating 18 cisterns for the collection of rainwater: in the event of a prolonged siege, they could make the community entrenched on the promontory self-sufficient. This cistern, the largest one, was obtained from a pre-existing natural cavity, waterproofed with mortar and covered with a brick barrel vault. Inside, a series of steps slowed the fall of the water, thus keeping it clear: it was then drawn by means of a pulley hanging from a beam.