Complex sentence: Born in Toledo, Spain, Alfonso was the eldest son of Ferdinand III of Castile and Elizabeth (Beatrice) of Swabia, through whom he was a cousin of Frederick II, Holy Roman Emperor, to whom Alfonso is often compared.
Split into simpler sentences: Born in Toledo, Spain, Alfonso was the eldest son of Ferdinand III of Castile and Elizabeth (Beatrice) of Swabia. His mother was the paternal cousin of Holy Roman Emperor Frederick II, to whom Alfonso is often compared.