Such was the strength of the fortress and the difficulties of its position that Ferdinand anticipated much trouble in reducing it, and made every preparation for a regular siege. In the centre of his camp were two great mounds, one of sacks of flour, the other of grain, which were called the royal granary. Three batteries of heavy ordnance were opened against the citadel and principal towers, while smaller artillery, engines for the discharge of missiles, arquebuses, and crossbows, were distributed in various places to keep up a fire into any breaches that might be made, and upon those of the garrison who should appear on the battlements. The lombards soon made an impression on the works, demolishing a part of the wall and tumbling down several of those haughty towers which, from their height, had been impregnable before the invention of gunpowder. The Moors repaired their walls as well as they were able, and, still confiding in the strength of their situation, kept up a resolute defence, firing down from their lofty battlements and towers upon the Christian camp. For two nights and a day an incessant fire was kept up, so that there was not a moment in which the roaring of ordnance was not heard or some damage sustained by the Christians or the Moors. It was a conflict, however, more of engineers and artillerists than of gallant cavaliers; there was no sally of troops nor shock of armed men nor rush and charge of cavalry. The knights stood looking on with idle weapons, waiting until they should have an opportunity of signalizing their prowess by scaling the walls or storming the breaches. As the place, however, was assailable only in one part, there was every prospect of a long and obstinate resistance. The engineers, as usual, discharged not merely balls of stone and iron to demolish the walls, but flaming balls of inextinguishable combustibles designed to set fire to the houses. One of these, which passed high through the air like a meteor, sending out sparks and crackling as it went, entered the window of a tower which was used as a magazine of gunpowder. The tower blew up with a tremendous explosion; the Moors who were upon its battlements were hurled into the air, and fell mangled in various parts of the town, and the houses in its vicinity were rent and overthrown as with an earthquake. The Moors, who had never witnessed an explosion of the kind, ascribed the destruction of the tower to a miracle. Some who had seen the descent of the flaming ball imagined that fire had fallen from heaven to punish them for their pertinacity. The pious Agapida himself believes that this fiery missive was conducted by divine agency to confound the infidels--an opinion in which he is supported by other Catholic historians.* * Pulgar, Garibay; Lucio Marino Siculo, Cosas Memoral. de Hispan., lib.20.