Fernando III was born in August 1201 in the forest between Salamanca and Zamora as the third child and eldest son of King Alfonso IX of Leon - the north-west corner of the Iberian peninsula - and his second wife Berenguela of Castile. Berenguela was born in 1180 as the eldest child of Alfonso VIII of Castile and his queen Eleanor of England, the second daughter of Henry II and Eleanor of Aquitaine, while Alfonso IX, known as el Baboso or the Slobberer because he foamed at the mouth during his frequent rages, was born in 1171 and succeeded his father as king of Leon in 1188. Fernando had two older half-sisters and a half-brother from his father's first marriage, four full siblings and at least a dozen, maybe as many as fifteen, half-siblings, his father's illegitimate children - one of whom, Maria, supposedly had an affair with her half-nephew, Fernando's son Alfonso X. Fernando himself had fifteen children, eleven sons and four daughters, all of them legitimate; most unusually for a Spanish king of the era, he is not known to have had any out-of-wedlock children. (Well, he was a saint, after all.)
Berenguela of Castile and Alfonso IX of Leon were first cousins once removed but didn't bother to apply for a dispensation on the grounds of consanguinity, and Pope Innocent III annulled their marriage in 1204 - although he did allow their children to remain legitimate. Alfonso, somewhat bizarrely, returned to his first wife Teresa of Portugal, who was his first cousin; the pope had previously annulled this marriage for consanguinity, too. Fernando III's elder half-brother, confusingly also called Fernando and the son of Alfonso IX and Teresa, died in August 1214 in his early twenties, leaving Fernando as the heir to his father's kingdom of Leon. A few weeks later, Fernando's maternal grandfather Alfonso VIII of Castile died, to be succeeded by his youngest child but only surviving son, ten-year-old Enrique I. (Fernando was three years older than his uncle.) Enrique reigned as king of Castile for less than three years, and was killed at Palencia by a tile falling off a roof in June 1217. To make this complicated family tree even more complicated, Enrique was married to Mafalda, sister of Teresa of Portugal.
Fernando's mother Berenguela, eldest child of Alfonso VIII and regent for her young brother from 1214 to 1217, succeeded as queen of Castile in her own right, but immediately abdicated in favour of her son. At not quite sixteen, Fernando was now king of Castile, and also succeeded his father as king of Leon on the latter's death in 1230. Alfonso IX tried to leave his kingdom to his daughters by Teresa of Portugal, Fernando's half-sisters Sancha and Dulce, but the two women were no match for Fernando and his determined, capable mother and chief adviser Berenguela (sister of the famous and equally capable queen-regent of France, Blanche of Castile).
Fernando married his first wife Elisabeth or Beatriz of Swabia in 1219, when he was eighteen and she fourteen. Remarkably, Beatriz was the granddaughter of two emperors: Frederick Barbarossa, Holy Roman Emperor, and Isaac Angelos, Byzantine Emperor. She bore Fernando three daughters and seven sons, including: his successor Alfonso X; Fadrique, executed for rebellion against Alfonso; Sancho, archbishop of Toledo at eighteen; and Felipe, archbishop of Seville also at eighteen, who gave up his ecclestiastical career to marry the Norwegian woman betrothed to one of his brothers. Fernando's most colourful child, however, was Enrique (1230-1304) who was at various times a mercenary in North Africa, a senator of Rome and the regent of Castile, spent thirty years in a Naples prison and four years in England cheerfully sponging off Henry III after his expulsion from Castile following an unsuccessful rebellion against his brother Alfonso X, and who was said - probably apocryphally - to be the lover of his stepmother, Edward II's grandmother Jeanne de Dammartin. Queen Beatriz died in 1235, aged only thirty - probably worn out from all that childbearing - and in 1237 Fernando married Jeanne de Dammartin, countess of Ponthieu, Montreuil and Aumale in her own right, in a match arranged by his aunt Blanche, queen of France. Fernando and Jeanne had four sons and a daughter, Edward II's mother Leonor or Eleanor, the only one of Queen Jeanne's children to outlive her.
Fernando is chiefly remembered for his great military successes, and he recaptured Cordoba, Jaen and Murcia from the Moors between 1236 and 1246. The emir of Granada paid tribute to Fernando as his vassal from 1246, though the king's greatest triumph was his conquest of the great city of Seville, which had been in the hands of the Moors for 536 years: after a sixteen-month siege, the king entered the city on 22 December 1248. I don't know for sure if she was, but I'd like to think that Edward II's mother Leonor, then probably aged seven, was present to witness the greatest moment in her father's life. Fernando more or less completed the Reconquista, with only Granada remaining in Muslim hands - and so the situation remained until 1492, when Ferdinand of Aragon and Isabella of Castile defeated the emirate and annexed it to Castile.
According to one report, Fernando mocked the Muslim inhabitants of Seville in 1248 by riding his horse up the Giralda tower, the minuet of Seville’s Great Mosque (which he later converted into a cathedral), perhaps one of the factors which prompted a Muslim writer to describe him as "the tyrant, the cursed one." In fairness, however, Fernando is remembered for his role in the Convivencia, the peaceful co-existence of Christians, Jews and Muslims in Spain, and he asked the pope to end the practice of Jewish people being forced to wear distinguishing marks on their clothes. Fernando founded the cathedral of Burgos as a very young man in 1221 and turned the mosque (mezquita) of Cordoba into another cathedral, founded bishoprics, hospitals and monasteries, and became famous for his great piety - he often fasted, spent entire nights praying, and always wore a hairshirt. Ever the soldier, however, he had a famous sword named Lobera.
Fernando III of Castile and Leon died in Seville on 30 May 1252, at the beginning of his fifties, to be succeeded by his thirty-year-old son Alfonso X; his much younger widow Queen Jeanne returned to her native Ponthieu and outlived him by twenty-seven years. His daughter Eleanor/Leonor, then aged ten and the twelfth of his fifteen children, was present at his death-bed. He was canonised 419 years after his death with his feast day as 30 May, and is the patron saint of engineers, soldiers, prisoners, paupers, people in authority in general, and large families. His first cousin Louis IX of France - their mothers were sisters - was also canonised as St Louis, and his elder half-sister and her mother Teresa of Portugal were beatified: a very saintly family, evidently. Here's a picture of the statue of Fernando on Plaza Nueva in Seville, and here's an account of a tourist celebrating the feast of San Fernando in 2006, with a photo of Fernando's body - his tomb in Seville Cathedral is opened every year, and his body is said to be incorruptible. Fernando III was a superb military leader, a tolerant man of great integrity and a renowned upholder of justice; qualities which, sadly, were not inherited by his English grandson Edward II.