The Fortress 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. Escrivà planned a giant fortress, made of four bastions connected through mighty walls, 60 meters long, with a thickness of 30 metres at the bottom and 5 meters at top. The walls were surmounted by massive merlons, with openings for the archers and the long-distance cannons. All around the fortress was a ditch (never filled with water) 23 meters wide and 14 meters deep, aimed at defending the foundations from the enemy's artillery; the slanted walls would reject enemy fire to the sides; each bastion consisted of two separate and completely self-sufficient environments - called "case matte" - almost independent garrisons on their own. Also the aqueduct to the city was deviated so as to supply the fortress first of all.
Moreover, Don Pirro planned a special anti-mine corridor, a kind of empty space between the outer and inner walls which could be walked only by one man at a time (and which can be visited today), aiming at defending the castle in case of explosion in case enemy soldiers excavated tunnels to leave mines at the foundations. A whole hill was leveled down to supply the white stone necessary for the fortress, while the city's bells were melted to make the cannons.
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. Between 1949 and 1951 the castle was restored, and chosen as the seat of the Museo Nazionale d'Abruzzo.