Alfonso Fernández de Castilla Alfonso Fernández " el Niño " ( b . c . 1243 - 1281 ) was a Spanish noble , the illegitimate son of Alfonso X of Castile , King of Castilla and León , and of Elvira Rodríguez de Villada . He was the Señor de Molina and of Mesa through his marriage to Blanca Alfonso de Molina , daughter of the infante Alfonso of Molina , and niece of King Alfonso IX of León . Family Origins . He was the son of Alfonso X of Castile , King of Castile and León , and Elvira Rodríguez de Villada . His paternal grandparents were Ferdinand III of Castile , King of Castile and León , and his wife , Queen Elisabeth of Swabia . His maternal grandparents were Rodrigo Fernández de Villada and his wife , María Muñoz . He was the step brother of , amongst others , Sancho IV of Castile , King of Castile and León , the infante Ferdinand de la Cerda , and of the infante Juan de Castilla . Biography . Alfonso ' s exact date of birth is unknown , although many historians consider it to be around the year 1243 . From 1255 he appears in the historic record as a confirmed benefactor of certain royal privileges . He always enjoyed the confidence of his father , King Alfonso X of Castile due to his ability and competence , demonstrated in both the political and military spheres of royal duties . In 1262 , when he was around 19 years of age , he was charged with presiding over the commission for the division of land of the conquered Taifa of Niebla , expropriating land for the King ' s brother , Infante Henry of Castile with the help of Pedro Lorenzo , the Bishop of Cuenca . Five years later , in April 1267 , he worked with Ferrand García , Archdeacon of Niebla , and the royal scribe , Domingo Ruiz , in the territorial demarcation of Huelva , Isla Saltés , Niebla , and Gibraleón . They were aided in this task by various caballeros , hombres buenos , and Muslim experts who knew the land and the villas to be demarcated . After the defeat of the Mudéjar s of Andalucia and Murica , who had risen in revolt against King Alfonso X in 1264 , Alfonso Fernández stayed in the area around Jerez de la Frontera . On 12 October 1269 , Alfonso X ordered that there could be no more than 30 donadíos in that city and that the rest of its land would pass to the population of the city . Later in the year , with advice from Diego Sánchez de Funes amongst others , he participated in the demarcation of Medina - Sidonia . In 1272 , Alfonso of Molina died and Alfonso Fernández , along with his wife , inherited the Señorio of Molina and of Mesa , although it was his wife who really administrated the Señorio as Alfonso Fernández was given the responsibility of being the tenant - in - chief of Sevilla and spent most of his time in Andalucía . Later in the same year , he was given a greater privilege with the title Señor de Molina and as son of the King . As the position of Adelantado Mayor of the Frontier of Andalucia was vacant at the time , Alfonso was charged with many of the duties associated with that office , although he is not mentioned with that title in any official documents from that time . In 1273 , being regarded as an expert on military strategy , he accompanied his step brother , the infante Ferdinand de la Cerda , son and heir of King Alfonso X , to the campaign that began in that year against the Emirate of Granada and against the rebels that had risen up against the King . Amongst others who joined this campaign were the infante Felipe de Castilla y Suabia , brother of the king , and Nuño González de Lara " el bueno " , the then head of the House of Lara . In 1274 , his father entrusted him with the defence of Sevilla while he pursued the " Fecho del Imperio " or the title of King of the Romans , the throne of the Holy Roman Empire . While Alfonso Fernandez was defending the city against Muslim attacks , the Castillian troops were defeated at the Battle of Écija and the commander of the Christian troops and head of the House of Lara , Nuño González de Lara " the good " , was killed during the battle . In 1278 , he participated , along with another step brother , the infante Pedro de Castilla y Aragón , son of Alfonso X , and Sancho IV of Castile , the future king , in the command of the Siege of Algeciras . The siege eventually failed the next year and occurred at the same time as the naval disaster at the Battle of Algeciras which saw the complete destruction of the Castillian fleet . Alfonso Fernandez found himself in charge of the vanguard of the column of Castilian - Leonese forces during the retreat from Algeciras . In 1281 , he participated , along with the King , and the infantes Sancho , Pedro and Juan in an expedition of punishment and attrition against the Emirate of Granada . Alfonso Fernandez was put in charge of a column situated in the rearguard of the army . The other columns were commanded by the king and the three other infantes . Death . Alfonso Fernández " el Niño " died later in 1281 , just after finishing the campaign against Vega de Granada . His last historical mention was in a letter written by his step brother Juan dated 26 August 1281 . Sepulcher . After his death , his body , according to various authors , was buried at the Monastery of Santa María de Matallana which is located in the Province of Valladolid . The claim of Alfonso ' s burial at that monastery was denied however by the Marqués de Mondéjar , Gaspar Ibañez de Segovia Peralta y Mendoza in his book " Memorias historicas del Rei D . Alonso el Sabio i observaciones a su chronica " . The Monastery of Santa María de Matallana is today a ruin and unfortunately nothing has been conserved of the mortal remains of Alfonso Fernandez nor his sepulcher . Manuel Gómez - Moreno , in his work " Sepulcros de la Casa Real de Castilla " , does not mention Alfonso Fernandez as having been buried in any particular place , although he does mention that he died in 1281 , shortly after the military expedition to Granada . Marriage and descendants . Alfonso Fernandez married Blanca Alfonso de Molina , who held titles over Molina and Mesa . She was the daughter of Alfonso of Molina and his wife , Mafalda González de Lara , they had two children :