There were considerable differences between the Hispanic kingdoms within the Iberian peninsula. It was divided by language and religion, governed by different political institutions and ruled by monarchs with conflicting foreign interests. Both Aragon and Castile had very little in common. The economies varied in strength in different parts of Spain, for example Catalonia was in decline whilst Valencia was in the recovery stage. The marriage of Isabella and Ferdinand in 1469 dramatically changed the course of Spain.
The Queen was a determined and quick-witted woman, with a 'street-fighter's instinct for survival'. She guaranteed privileges to nobles and town authorities, promised to restore law and order and was the only claimant with an undisputed right to the throne. Ferdinand played a role in leading Isabella's troops against the Portuguese and French invasions and recaptured rebellious towns, all helping to stabilise the crown. His brother also aided Isabella by commanding the Hermandad meaning that although his father died, no Aragonese nobles contested the crown. Royal authority had declined during the weak rule of Henry IV and it needed to be asserted and there was also the problem of law and order (lots of disorder in Castile due to the 5 years civil war).
There were still supporters of Joanna; a minority of the nobility had fought against Isabella e.g Marquis of Vienna. The Hermandad at least brought some justice to the countryside, which benefited Castilian towns as well as law-abiding landowners. Hermandades had never been controlled so extensively by the Crown, it significantly contributed to the improvements in local administration. Corregidores were also having an impact (administrators of cities and districts with both administrative and judicial powers).
They were used by the Catholic Monarchs to strengthen royal authority rather than revive local responsibility Ferdinand and Isabella travelled extensively in Castile. They spent relatively little time in Aragon in his 37-year reign (7 years). This was a very important means of asserting royal authority by showing themselves to their subjects and establishing an effective working relationship with their nobility and to discipline any troublemakers. Ferdinand and Isabella visited every Castilian town at least once.
Ferdinand tried to increase royal authority over Aragonese towns by trying to introduce new procedures for the election of municipal offices. His idea of sending royal governors to towns was rejected preventing royal authority encroaching. Isabella was able to have a greater effect on improving local administration than her husband. She tried to protect, by intervention, town people from impressive nobles and nominated jurados to sit on town councils.
She persisted with the idea of Corregidores in towns, despite opposition, and the number of towns with them increased (Toledo for instance enjoyed a long period of peace largely due to Gomez Manrique, its Corregidore.). Their authority varied, some governed entire regions, and others had a limited impact. However, overtime they became more corrupt lowering the standards of local administration.
The civil war left deep scars and required close supervision to work effectively. Both Isabella and Ferdinand dispersed justice personally. The Hermandad was unpopular and it demanded taxation on local people for its maintenance. It was officially dissolved in 1498 as nobles increasingly disliked it (didn't want too much crown influence). The crowns political strength lay into relationship with the nobility.
Ferdinand and Isabella however, due to incompatible promises in the 1470's, were in a different situation as some families expected rewards for their role in the civil war. This often contradicted what Isabella had said about protecting the rights of the towns. Isabella decided to harness the power of the nobility and tried to regain land given out by Henry IV. Isabella and Ferdinand on a smaller scale in Aragon gave the nobles rights to collect sales tax and confirmed their exemption from direct tax. This guaranteed their social and economic position.