This Gunpoweder Magazine was built in the 17th century. The storage of gunpowder was a risky undertaking that required adequate and safe spaces free from the risk of fire and bombardment, and adequate protection against spoilage from dampness and rain water.
Up until the sixteenth and seventeenth centuries, there were no established forms of structures designed to serve solely as gunpowder magazines. Any ordinary and available building, preferably dry, was put to use as a storage are when the need arose.
The gunpowder was generally kept in wooden barrels stacked horizontally in rows on wooden skids, or tavolate.
The walls were sometimes padded (infoderate) to help reduce the effects of rising dampness through the walls. Blind ventillation shafts, known as sfiatatori, ensured that no material could be introduced into the interior of the magazine from outside. The external or internal openings were fitted with wooden apertures.
It was also neccessary for the powder magazines to be located away from built-up areas, whether civilian or military, as a safety precaution. Powder magazines needed to be inspected regularly by the commander of artillery or his subordinate to ensure that they remained capable of housing gunpowder in suitable conditions.
References: Stephen C. Spiteri, The Art of Fortress Building in Hospitaller Malta, Malta, 2008, pp. 489, 498, 501, 502)
Source: Wirt Ghawdex