Don Pedro of Toledo succeeded Filiberto and decided to entrust the fortress project to a celebrated Spanish architect, Don Pirro Aloisio Escriva, a great expert of firearms, who had begun to build Castle Sant'Elmo in Naples. The discovery of gunpowder obliged to new methods of defensive construction. Escribà was in charge of the project 2 years, leaving the task to Gian Girolamo Escribà.
In the following 30 years the heavy taxes impoverished the city, which in 1567 begged the Spaniards to stop the construction of the castle; the Royal Court granted the request, and works were interrupted, so some parts of the castle were never completed. The Fortress had cost an enormous sum for the times, and Aquila was obliged also to sell the thick silver case containing the body of St. Bernardine from Siena.
The Fortress, which had been built not to defend the city, but to control it (many cannons pointed to the city) and to be a completely self-sufficient structure, was never used in a battles. Its cannons, always ready to fire, were silent throughout the centuries: the only victim was the city itself, whose decline began with the construction of the fortress and went on under the Spanish dominion.