This vast underground vaulted hall was part of the fortified castle built in 1514. It was probably a weapons room before being used as a water reservoir. Its square plan with 34m sides is composed of six naves with groined vaults resting on 25 columns and pillars. The central bay is pierced by a wide oculus through which daylight streams in, producing a surprising mirror effect by reflecting on the cistern water, imbuing the place with a strange atmosphere.