The Gran Prigione (Grand Prison[3]) was established around 1585[4] during the magistracy of Hugues Loubenx de Verdalle (1582–95).

[6] The building served as the Order of St. John's main prison, as well as a compound in which slaves were locked up at night.

[8] A Turkish slave who had been a hydrologist in Constantinople reportedly helped in the construction of the aqueduct, and in recognition of his work he was given his freedom and the prison became one of the first buildings in Valletta to be supplied by running water.

[11] After the plot was discovered and brutally suppressed, a decree was issued stating that all slaves were to be locked up at the prison at night.

[12] The prison was a large three-story building occupying an entire city block close to the Lower Barrakka Gardens.

[18] A mid-19th century report describes it as:[7] a lofty quadrangular building, standing on the brow of a hill fronting the Grand Harbour.