How did the Cortes of Spain differ from other organs of the estate monarchy you know? Why did this difference appear?

The Cortes of Spain are a kind of estate assembly, where each estate elected its representatives to express their opinions and protect the interests of the estate. These were regional meetings. It should be noted that initially only representatives of the nobility and clergy were included in these meetings. Only after a while the townspeople were allowed to participate in the meetings.
It is worth noting the historical fact that the first Cortes appeared in Castile. Founded in 1137 AD.