The Middle Ages were an extremely lengthy period of time, spanning some one thousand years, during which many things changed over the course of generations. To provide a comprehensive answer to the many ramifications of such question is difficult, but I will try and provide some guidelines on the matter.

First and foremost, rulers during the Middle Ages did not possess absolute authority over their subjects. Taxes could not be raised or instated on a whim, people could not be punished by wish, property could not be confiscated without effort. Virtually at every level of society, but more clearly at the higher ranks of it, law and custom were paramount in defining how power should be applied.

You may be familiar with the phrase: "No taxation without representation". An analogy of its significance can be found in the numerous summons of councils in various Western European kingdoms between the XIII and XVI centuries. Taking the kingdom of Naples, both Angevin and Aragonese, as an example, king Charles of Anjou, during the War of the Sicilian Vespers (1282-1302), called several parlamenti ("parliaments") where all vassals, feudal lords and prominent cities of the realm were called to participate to the negotiation of extra-ordinary taxes and economic contribution to the king's war.

Jumping ahead some 150 years, king Alfonso I of Aragon called in several parliaments, fifteen to be exact, between 1443 and 1458. We have a wealth of informations regarding the first and the 1454 one. We know that for the former, the letters of summon bore the motivation: «[...] for the sake of public well-being ("pro bono rei puplice") [...] and for the conservation and exaltation of our royal status», whilst the latter had the sentence: «to provide to the defense of said realm» (my translation). These assemblies were mainly dedicated to the negotation of taxes, tolls and other economic matters, usually grievious or unpopular ones which required confrontation between all elements of society (although, to be fair, not in all parliaments aristocracy, urban communities and the clergy were present all at once). They were used to quell unrest and rebellion, as we are aware about the parliament of 1485 when rebellious barons met king Ferrante (Alfonso's bastard son) in Miglionico, but also to provide a formal space to channel the requests of all those who attended the meeting, and some historians have speculated a form of pact based on political-financial needs reciprocated between the royal authority and the other powers of in the kindgom.

It wasn't always this clear however, as we know that in 1484's parliament, the duke of Andria, (king Ferrante's son) was the one who had been chosen before the start of the assembly to coordinate the parliament's response. We also know that the city of Gaeta - a major coastal and fluvial trade hub and naval base - was granted a privilege in 1482 because in the previous year's parliament: «to give good example to the other cities, the city of Gaeta accepted what had been decreed by king Ferrante during the general parliament held in Naples. As a reward for this, the duke of Calabria promised her a special favour».

Another example is the Regia Camera della Sommaria ("Royal Chamber of Summaries"), the highest financial, administrative and judicial body in the Aragonese kingdom who had the last word about all matters regarding money and administration. Revenues and exemptions had to be examined and confirmed by the Sommaria. In 1484, king Ferrante managed to convince pope Sixtus IV to split ecclesiastical revenues in the kingdom of Naples (about 18/25 to the royal fisc and the rest for the papacy). A monastery which was held by the son of a count chose not to pay, and as they were pressed into compliance by royal officials, the abbot provided a document (a "priviledge" as it is known in our field) dating back to 1404 which stated that that monastery could not be accounted for in fiscal matters. The Sommaria ruled that they were exempt.

One last example is the presence of royal officials in the cities of the kindgom during all of its existence. Cities still elected their own representatives based on local custom, which often revolved around localized smaller councils of aristocratic families related by blood called seggi ("seats"). Alongside them were royal officials which had several names, Naples had one called stratigotus, which may have had a Greek root from Byzantine days, others had capitani ("captains") which confronted the urban elected officials. The city of Sessa, whose dukes were Grand Admirals of the kingdom of Naples and whose last duke, Marino Marzano, was married to king Ferrante's half sister Eleonora of Aragon, had, as of 1464, the right to elect three mayors among the most prominent families which would stay in charge for one year alongside the royal captain appointed by the king. This is supposed to have been due the prominence of urban centers in the politics of the kingdom. It wasn't uncommon that the presence of a captain, thus being within royal demesne, was a priviledge granted to centers of conspicous importance, like the aforementioned Sessa which was a fortified city on the northern borders and commanded several mountain passes.

I hope this helps your inquiry. Feel free to ask more.
Sources:
Vitolo, G. 2014, L'italia delle altre città. Un'immagine del Mezzogiorno medieval
Senatore, F., Scarton E. 2018, Parlamenti generali a Napoli in età aragonese, FedOA Press;
Massaro, C. 2011, Fiscalità pontificia e regno di Napoli nel secondoQuattrocento. Due registri di decime di Sisto IV, in Scritti di storia medievale offerti a Maria Consiglia de Matteis, Fondazione CISAM;
Broccoli, A. 1889, Codice Municipale Sessano, in Archivio Storico Campano, Stabilimento Tipografico Sociale.