This is a difficult question to answer. While Iberian expansionism can largely be attributed, in its inception, to the institutional stances of the monarchies of Castile and Aragon (and Portugal too, since we're talking about Iberia) the characteristics and consequences tied to the Spanish colonization of the Americas, as with the colonization of other places across the world, were very different than those tied to the Spanish presence and impact in Italy.

Indeed the Spanish colonization of the Americas has consequences so profound and far-reaching, it is easy to argue that it is one of the great landmark events of world history (or at least, it achieves this distinction as part of the broader topic of European colonization of the Americas). Similarly, the Spanish presence in Asia and Africa had far-reaching and lasting consequences for both the Spanish and the communities they interacted with. Can the same be said of the Spanish presence in Italy? Probably not in the same way.

I'm not a historian of Iberia, nor do I know all that much about the Americas before the 20th century (and even then, my knowledge is mostly tied to economics). But I do know a thing or two about Italy. As would be demonstrated most prominently in the Americas, Spanish expansionism was most successful where Spanish political decision makers could understand the existing political and social dynamic. Italy was one such place, where social interactions between Spanish and Italian economic and cultural leaders were already well-developed along the Mediterranean sea-lanes. This was especially true for the subgroups of Aragonese and Sicilians, as we will see below. Both the Spanish and Italian leadership was Catholic, both spoke languages with some degree of mutual intelligibility, and both shared the Latin cultural canon and education.

The Spanish presence in Italy traces its origins to the War of Sicilian Vespers, a conflict for the nominal control of Italy's largest island which was resolved in favor of the Crown of Aragon. Leading Sicilian aristocrats had internalized and accepted the end of a Kingdom of Sicily that unified the whole of Southern Italy, and that conflict which saw the Sicilian crown passing to the aragonese monarch pitted a multitude of social forces against each other in order to forcibly create a new order: old landowners with ties to mainland Italy aligned themselves against rising aristocrats investing in cross-Mediterranean economic ventures, old baronial interests sought to curb the growing wealth and influence of the cities, and a rigid social system ill-fitting for a changing social and political landscape was being challenged from all sides.

The rest of mainland Southern Italy would fall into the aragonese Kingdom's fold in a second, later moment following a similar political crisis at the start of the 15th century. After a brief but colorful period where a new Kingdom of Naples was founded and governed by a cadet branch of the Aragonese monarchy, the whole south of Italy would eventually become a viceroyalty of newly unified Spain.

In all instances of conflict and eventual Aragonese (or Spanish) imposition of control in Southern Italy, the collaboration of the local aristocracy was instrumental. Both due to official decisions like the formalization and retention of generous rights and privileges enjoyed by the aristocracy, to the less formal reliance on the aristocracy to be the monarchy's eyes and ears in the country, the ultimate result of Spanish governorship of southern Italy was a stratified and deeply unequal society favoring titled landowners. With a viceroyal presence limited to the city of Naples, which would balloon to enormous size, and a smaller presence in a few other select cities, aristocratic landowners in Southern Italy were in positions of power unparalleled in the rest of the peninsula. The urban middle class remained small, much smaller than its counterparts in the rest of the peninsula, not in the lease because save for the gargantuan Naples and Palermo, the South's cities themselves remained small, creating few opportunities for internal trade and even fewer chances for external trade.

The segmentation of Southern Italian society is of course not wholly the fault of Spanish rule. Had an autochthonous monarchy survived, it is possible that Southern Italy's isolation would have pushed for aristocratic entrenchment regardless. Indeed, the Southern Kingdom had only one very stable land border to worry about, allowing for self-referential introspectiveness and internal entrenchment. But it's also true that Spanish rule existed elsewhere in Italy and developed very different implications: The Duchy of Milan, for example, would become a key point of both hard and soft power projection into Central Europe. Very tight economic collaboration also developed with the Republic of Genoa, entwining the fate of the city's seafaring entrepreneurial class with that of the Spanish Empire. And importantly, none of these polities exhibited the sort of social entrenchment and stratification which developed in Southern Italy.

I don't know if I offered a conclusive answer to your question. I suppose the point would be that while Spanish rule certainly had an impact on Southern Italy, this impact was largely self-contained. It did, however, create lasting implications for the social and political development of the South.

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