Your analysis checks out. https://www.thoughtco.com/castillo-last-name-meaning-and-origin-1422471 >>Where is the CASTILLO Surname Most Common? Castillo is the 232nd most common surname in the world, according to surname distribution data from Forebears. It is a prevalent name in many Spanish-speaking countries, including Mexico (the 25th most common last name), Venezuela (14th), Peru (18th), Guatemala (11th), Dominican Republic (14th), Chile (19th), Ecuador (25th), and Spain (44th). >>WorldNames PublicProfiler doesn't include data from as many countries but does include both Argentina and Spain, where the Castillo surname is very popular. In Spain, Castillo is found in the greatest numbers in the Cantabria region, followed by Andalucia, Castilla-La Mancha, and La Rioja. Castillo is also very prevalent in the Gran Chaco, Patagonia, and Argentine Northwest regions of Argentina.