As brought to our attention by Spanish fansite Los Siete Reinos, at least two new locations in Spain are lining up for filming in season 6, which starts filming this July but the Spanish segment appears set for September and October. According to Barcelonan paper La Vanguardia, HBO has apparently confirmed that the Catalonian city of Girona will be used in filming—its famous cathedral being cited as one location of particular interest—as well as the Valencian medieval town of Peñsicola which is known for retaining many medieval and Renaissance buildings and structures. James Hibberd of EW adds that other locations may be used in Spain, though those have as yet to be confirmed (in particular, James Hibberd notes that there’s no word whether Seville, the main site for filming Dornish scenes, has not been confirmed as a filming location at this time).
Both Los Siete Reinos and La Vanguardia speculate that Girona would serve very well for Oldtown and the Citadel, citing the narrow streets of the Jewish quarter, the edifice of the cathedral, and so on. Peñsicola is more of a mystery, although Spanish fans are tentatively suggesting it could serve for Casterly Rock. In support of this, note that the Wikipedia article notes that Peñsicola is sometimes known as the “Gibraltar of Valencia”, thanks to being a prominent fortified peninsula, and George R.R. Martin has often likened Casterly Rock to the actual Gibraltar, while at the same time he has noted that Casterly Rock will appear in future A Song of Ice and Fire novels.
It’s certainly as good a guess as any.