In 1516 the nephew Charles V succeeded the Catholic Ferdinand in the Kingdoms of Aragon, Castile and Sardinia, who after three years became emperor. Cagliari welcomed the new sovereign in 1535, who arrived with a large fleet in tow and, to celebrate this important historical event, an inscription indicating six hundred ships was placed on the facade of Palace of the City, the first seat of the town hall. (But from other sources it emerged that they docked in just over four hundred).

Charles V wanted to strengthen and modernize the fortifications of Cagliari and entrusted the task to the military engineer and architect Rocco Cappellino who, in almost twenty years of stay, built a powerful system consisting of walls and ramparts.