Spain still uses the Roman Law and save for the Basques (which are the cousins of Iberians) the 95% of the culture it's Western. The 5% it's just pre-Roman folklore (Basque, Celtic, Celtic-Iberian and so on) which survived Romanization and Christianization. But, even when being fully assimilated, I can always find some older Iberian substrate surviving in the North of Spain and the French Basque Country, some social behaviour predating Rome and the Catholic Kingdoms, such as the concept of the communal assemblies in towns and villages found all over in Spain.

From these arrangements between the people and Kingdoms the concept of Fueros (some kind of agreement/constution between the villages' ruling and the King) was born and if some king was about to rule a Kingdom, he/she was prompty required to respect them 'by the grace of God' AKA 'respect these scrolls or you will be kicked from the throne faster than a drunk knight falling off from a horse'.