Author of the anthem "Gernikako Arbola." The song celebrates the Tree of Gernika and the Basque liberties and is an unofficial anthem of the Basque people. In the Middle Ages, representatives of the villages of Biscay would hold assemblies under local big trees. As time passed, the role of separate assemblies was superseded by the Guernica Assembly in 1512, and its oak would acquire a symbolic meaning, with actual assemblies being held in a purpose-built hermitage-house (the current building dates from 1833). It was the Spanish regent Maria Christina accompanied by her infant daughter Queen Isabella II the last Spanish monarch to swear an oath to the charters under the iconic oak in 1839.