In fact you can see it in the flag, it has a shield with the chains of Navarra, the lion of Leon, the Castle of Castilla and the colors of Aragon(which are the same of Spain and Catalunya flag) over a Granada fruit(for Granada conquest).
Basque was never a kingdom,either Galicia or Catalunya. Asturias was a kingdom, later becoming Asturias and Leon kingdom, then growing again and again against the muslims in Alandalus.
To say that Basque was a kingdom is an enormous lie that unfortunately is told in Basque country as they control their own education.
My grandfather was Basque, he spoke Batua, the original Basque language in his village. He could not speak complex abstractions or communicate with people in villages 40 kms away as every batua was different. The language was unified less than 100 years ago(with the abstractions simply being invented at this time)and called euskera, a recent invention. Now kids there are brainwashed being told that this was the language that people spoke 3000 years ago.
The places in Spain that after industrialization are rich are rewriting History in order to look better themselves, specially if you are nationalist.