Frederick II (26 December 1194 – 13 December 1250), called "Stupor mundi", the "wonder of the world" was one of the most powerful Holy Roman Emperors of the Middle Ages and head of the House of Hohenstaufen. Viewing himself as a direct successor to the Roman Emperors of Antiquity, he was King of the Romans from his papal coronation in 1220 until his death; he was also a claimant to the title of King of the Romans from 1212 and unopposed holder of that monarchy from 1215. As such, he was King of Germany, of Italy, and of Burgundy. At the age of three he was crowned King of Sicily as a co-ruler with his mother Constance, the daughter of Roger II of Sicily. His other royal title was King of Jerusalem by virtue of marriage and his connection with the Sixth Crusade.

He was frequently at war with the Papacy, hemmed in between Frederick's lands in northern Italy and his Kingdom of Sicily (the Regno) to the south, and thus he was excommunicated four times and often vilified in pro-papal chronicles of the time and since. Pope Gregory IX went so far as to call him the Antichrist. He was said to speak six languages: Latin, Sicilian, German, French, Greek and Arabic. By contemporary standards, Frederick was an uncommonly avid patron of science and the arts. He was also patron of the Sicilian School of poetry. His Sicilian royal court in Palermo, from around 1220 to his death, saw the first use of a literary form of an Italo - Romance language, Sicilian. The poetry that emanated from the school had a significant influence on literature and on what was to become the modern Italian language. The school and its poetry were well known to Dante and his peers and predate by at least a century the use of the Tuscan idiom as the elite literary language of the Italian peninsula. After his death his line quickly died out and the House of Hohenstaufen came to an end. He was the last of the powerful Holy Roman Emperors.

Born in Jesi, near Ancona, Italy, Frederick was the son of the emperor Henry VI. He was known as the puer Apuliae (son of Apulia). Some chronicles say that his mother, the forty year old Constance, gave birth to him in a public square in order to forestall any doubt about his origin. Frederick was baptised in Assisi. In 1196 at Frankfurt am Main the infant Frederick was elected King of the Germans. His rights in Germany were disputed by Henry's brother Philip of Swabia and Otto of Brunswick. At the death of his father in 1197, the two year old Frederick was in Italy travelling towards Germany when the bad news reached his guardian, Conrad of Spoleto. Frederick was hastily brought back to his mother Constance in Palermo, Sicily. Constance of Sicily had been in her own right queen of Sicily; she had Frederick made King of Sicily and established herself as regent. In Frederick's name she dissolved Sicily's ties to Germany and the Empire that had been created by her marriage, sending home his German counsellors (notably Markward von Annweiler and Gualtiero da Pagliara) and renouncing his claims to the German throne and empire. Upon Constance's death in 1198, Pope Innocent III succeeded as Frederick's guardian. Frederick's tutor during this period was Cencio, who would become Pope Honorius III. Frederick was crowned King of Sicily on 17 May 1198.

Otto of Brunswick had been crowned Holy Roman Emperor by Pope Innocent III in 1209. In September 1211 at the Diet of Nuremberg Frederick was elected in absentia as German King by a rebellious faction backed by Innocent, who had fallen out with Otto and excommunicated him. Frederick was elected once again in 1212 and crowned 9 December 1212 in Mainz; yet another coronation ceremony took place in 1215. Frederick's authority in Germany remained tenuous, however, and he was recognized only in southern Germany; in northern Germany, the center of Guelph power, Otto continued to hold the reins of royal and imperial power despite his excommunication. But Otto's decisive military defeat at Bouvines forced him to withdraw to the Guelph hereditary lands where, virtually without supporters, he died in 1218. The German princes, supported by Innocent III, again elected Frederick king of Germany in 1215, and he was crowned king in Aachen on 23 July 1215 by one of the three German archbishops. It was not until another five years had passed and only after further negotiations between Frederick, Innocent III, and Honorius III — who succeeded to the papacy after Innocent's death in 1216 — that Frederick was crowned Holy Roman Emperor in Rome by Honorius III on 22 November 1220. At the same time Frederick's oldest son Henry took the title of King of the Romans.

Unlike most Holy Roman emperors, Frederick spent few years in Germany. In 1218 he helped Philip II of France and Eudes III, Duke of Burgundy to bring an end to the War of Succession in France Champagne by invading Lorraine, capturing and burning Nancy, capturing Theobald I, Duke of Lorraine, and forcing him to withdraw his support from Erard of Brienne. After his coronation in 1220, Frederick remained either in the Kingdom of Sicily or on Crusade until 1236, when he made his last journey to Germany. He returned to Italy in 1237 and stayed there for the remaining thirteen years of his life, represented in Germany by his son Conrad.

In the Kingdom of Sicily, he built on the reform of the laws begun at the Assizes of Ariano in 1140 by his grandfather Roger II. His initiative in this direction was visible as early as the Assizes of Capua (1220) but came to fruition in his promulgation of the Constitutions of Melfi (1231, also known as Liber Augustalis), a collection of laws for his realm that was remarkable for its time and was a source of inspiration for a long time after. It made the Kingdom of Sicily an absolutist monarchy; it also set a precedent for the primacy of written law. With relatively small modifications, the Liber Augustalis remained the basis of Sicilian law until 1819.

At the time he was elected King of the Romans, Frederick promised to go on crusade. He continually delayed, however, and, in spite of his renewal of this vow at his coronation as the King of Germany, he did not travel to Egypt with the armies of the Fifth Crusade in 1217. He sent forces to Egypt under the command of Louis I, Duke of Bavaria, but constant expectation of his arrival caused papal legate Pelagius to reject Ayyubid sultan Al-Kamil's offer to restore the Latin Kingdom of Jerusalem to the crusaders in exchange for their withdrawal from Egypt and caused the Crusade to continually stall in anticipation of his ever delayed arrival. The crusade ended in failure with the loss of Damietta in 1221. Frederick was blamed by both Pope Honorius III and the general Christian populace for this calamitous defeat.

Problems of stability within the empire delayed Frederick's departure on crusade. It was not until 1225, when, by proxy, Frederick had married Yolande of Jerusalem, heiress to the Kingdom of Jerusalem, that his departure seemed assured. Frederick immediately saw to it that his new father - in - law John of Brienne, the current king of Jerusalem, was dispossessed and his rights transferred to the emperor. Despite his new status as King of Jerusalem, however, Frederick continued to delay, and in 1227, Frederick was excommunicated by Pope Gregory IX for failing to honor his crusading pledge. In fact, Frederick had set out for the Holy Land but was forced to return when he was struck down by an epidemic that had broken out. Even the master of the Teutonic Knights, Hermann of Salza, recommended that he return to the mainland to recuperate. Many contemporary chroniclers doubted the sincerity of Frederick's illness, and their attitude may be explained by their pro-papal leanings. Roger of Wendover, a chronicler of the time, wrote ‘he went to the Mediterranean sea, and embarked with a small retinue; but after pretending to make for the holy land for three days, he said that he was seized with a sudden illness… this conduct of the emperor redounded much to his disgrace, and to the injury of the whole business of the crusade,’(‘Roger of Wendover’, Christian Society and the Crusades, ed Peters (Philadelphia 1971)). However, it is not surprising that his sincerity was viewed with skepticism, as he had delayed his departure for more than a decade after first taking the crusading vow.

He eventually embarked on the crusade the following year (1228). The pope regarded that action as a provocation, since, as an excommunicate, Frederick was technically not capable of conducting a Crusade, and excommunicated the emperor a second time. By this time the crusading army had dwindled to a meagre force. Knowing that he could not take Jerusalem by force of arms, Frederick negotiated along the lines of a previous agreement he had intended to broker with the Egyptian sultan, Al-Kamil. The treaty resulted in the restitution of Jerusalem, Nazareth, and Bethlehem to the Kingdom of Jerusalem, though there are disagreements as to the extent of the territory returned. The treaty also stipulated that the Dome of the Rock and al-Aqsa Mosque were to remain under Muslim control and that the city of Jerusalem would remain without fortifications. Virtually all other crusaders, including the Templars and Hospitallers, condemned this deal as a political ploy on the part of Frederick to regain his kingdom while betraying the cause of the Crusaders. The Ayyubid ruler of the region, who was nervous about possible war with his relatives who ruled Syria and Mesopotamia, wished to avoid further trouble from the Christians, at least until his domestic rivals were subdued.

The crusade ended in a truce and in Frederick's coronation as King of Jerusalem on 18 March 1229 — although this was technically improper. Frederick's wife Yolande, the heiress, had died, leaving their infant son Conrad as rightful king. There is also disagreement as to whether the 'coronation' was a coronation at all, as a letter written by Frederick to Henry III of England suggests that the crown he placed on his own head was in fact the imperial crown of the Romans. In any case, Gerald of Lausanne, the Latin Patriarch of Jerusalem, did not attend the ceremony, indeed, the next day the Bishop of Caesarea arrived to place the city under interdict on the patriarch's orders. Frederick's further attempts to rule over the Kingdom of Jerusalem were met by resistance on the part of the barons, led by John of Ibelin, Lord of Beirut. In the mid 1230s, Frederick's viceroy was forced to leave Acre, the capital, and in 1244, following a siege, Jerusalem itself was lost again to a new Muslim offensive.

Whilst Frederick's seeming bloodless recovery of Jerusalem for the cross brought him great prestige in some European circles, his decision to complete the crusade while excommunicated provoked Church hostility. Although in 1230 the Pope lifted Frederick's excommunication at the Treaty of San Germano, this decision was taken for a variety of reasons related to the political situation in Europe. Of Frederick's crusade, Philip of Novara, a chronicler of the period, said "The emperor left Acre [after the conclusion of the truce]; hated, cursed, and vilified." Overall this crusade, arguably the first successful one since the First Crusade, was adversely affected by the manner in which Frederick carried out negotiations without the support of the church. He left behind a kingdom in the Levant torn between his agents and the local nobility, a civil war known as the War of the Lombards.

The itinerant Joachimite preachers and many radical Franciscans, the Spirituals supported Frederick. Against the interdict pronounced on his lands, the preachers condemned the Pope and continued to minister the sacraments and grant absolutions. Brother Arnold in Swabia proclaimed the Second Coming for 1260, at which time Frederick would then confiscate the riches of Rome and distribute them among the poor, the "only true Christians".

While he may have temporarily made his peace with the pope, Frederick found the German princes another matter. In 1231, Frederick's son Henry (who was born 1211 in Sicily, son of Frederick's first wife Constance of Aragon) claimed the crown for himself and allied with the Lombard League. The rebellion failed, though not utterly; Henry was imprisoned in 1235, and replaced in his royal title by his brother Conrad, already the King of Jerusalem.

In Germany the Hohenstaufen and the Guelphs reconciled in the same year. Otto the Child, the grandson of Henry the Lion, deposed as Duke of Bavaria and Saxony in 1180, conveyed the allodial Guelphic possessions to Frederick, who in return enfeoffed Otto with the same lands and additional former imperial possessions as the newly established Duke of Brunswick - Lüneburg, ending the unclear status of the German Guelphs, who were left without title and rank after 1180. With peace north of the Alps Frederick won a decisive battle in Cortenuova over the Lombard League in 1237.

Frederick celebrated it with a triumph in Cremona in the manner of an ancient Roman emperor, with the captured carroccio (later sent to the commune of Rome) and an elephant. He rejected any suit for peace, even from Milan which had sent a great sum of money. This demand of total surrender spurred further resistance from Milan, Brescia, Bologna and Piacenza, and in October 1238 he was forced to raise the siege of Brescia, in the course of which his enemies had tried unsuccessfully to capture him.

Frederick received the news of his excommunication by Gregory IX in the first months of 1239 while his court was in Padova. The emperor responded by expelling the Minorites and the Preachers from Lombardy, and electing his son Enzio as Imperial vicar for Northern Italy. Enzio soon annexed the Romagna, Marche and the Duchy of Spoleto, nominally part of the Papal States. The father announced he was to destroy the Republic of Venice, which had sent some ships against Sicily. In December of that year Frederick marched over Toscana, entered triumphantly into Foligno and then in Viterbo, whence he aimed to finally conquer Rome, in order to restore the ancient splendours of the Empire. The siege, however, was ineffective, and Frederick returned to Southern Italy, sacking Benevento (a papal possession). Peace negotiations came to nothing.

In the meantime the Ghibelline city of Ferrara had fallen, and Frederick swept his way northwards capturing Ravenna and, after another long siege, Faenza. The people of Forlì (which kept its Ghibelline stance even after the collapse of Hohenstaufen power) offered their loyal support during the capture of the rival city: as a sign of gratitude, they were granted an augmentation of the communal coat of arms with the Hohenstaufen eagle, together with other privileges. This episode shows how the independent cities used the rivalry between Empire and Pope as a means to obtain maximum advantage for themselves.

The Pope called a council, but Ghibelline Pisa thwarted it, capturing cardinals and prelates on a ship sailing from Genoa to Rome. Frederick thought that this time the way into Rome was opened, and he again directed his forces against the Pope, leaving behind him a ruined and burning Umbria. Frederick destroyed Grottaferrata preparing to invade Rome. Then, on 22 August 1241, Gregory died. Frederick, showing that his war was not directed against the Church of Rome but against the Pope, drew back his troops and freed two cardinals from the jail of Capua. Nothing changed, however, in the relationship between Papacy and Empire, as Roman troops assaulted the Imperial garrison in Tivoli and the Emperor soon reached Rome. This back-and-forth situation was repeated again in 1242 and 1243.

A new pope, Innocent IV, was elected on 25 June 1243. He was a member of a noble Imperial family and had some relatives in Frederick's camp, so the Emperor was initially happy with his election. Innocent, however, was to become his fiercest enemy. Negotiations began in the summer of 1243, but the situation changed as Viterbo rebelled, instigated by the intriguing Cardinal Ranieri of Viterbo. Frederick could not afford to lose his main stronghold near Rome, and besieged the city. Many authorities state that the Emperor's star began its descent with this move. Innocent convinced him to withdraw his troops, but Ranieri nonetheless had the Imperial garrison slaughtered on November 13. Frederick was enraged. The new Pope was a master diplomat, and Frederick signed a peace treaty, which was soon broken. Innocent showed his true Guelph face, and, together with most of the Cardinals, fled via Genoese galleys to the Ligurian republic, arriving on July 7. His aim was to reach Lyon, where a new council was held beginning 24 June 1245. One month later, Innocent IV declared Frederick to be deposed as emperor, characterising him as a "friend of Babylon's sultan", "of Saracen customs", "provided with a harem guarded by eunuchs" like the schismatic emperor of Byzantium and, in sum, a "heretic". The Pope backed Heinrich Raspe, landgrave of Thuringia as his rival for the imperial crown and set in motion a plot to kill Frederick and Enzio, with the support of his (the pope's) brother - in - law Orlando de Rossi, another friend of Frederick's.

The plotters, however, were unmasked by the count of Caserta. The vengeance was terrible: the city of Altavilla, where they had found shelter, was razed, and the guilty were blinded, mutilated and burnt alive or hanged. An attempt to invade the Kingdom of Sicily, under the command of Ranieri, was halted at Spello by Marino of Eboli, Imperial vicar of Spoleto.

Innocent also sent a flow of money to Germany to cut off Frederick's power at its source. The archbishops of Cologne and Mainz also declared Frederick deposed, and in May 1246 a new king was chosen in the person of Heinrich Raspe. On 5 August 1246 Heinrich, thanks to the Pope's money, managed to defeat an army of Conrad, son of Frederick, near Frankfurt. But Frederick strengthened his position in Southern Germany, acquiring the Duchy of Austria, whose duke had died without heirs, and one year later Heinrich died as well. The new anti-king was William II, Count of Holland.

Between February and March 1247 Frederick settled the situation in Italy by means of the diet of Terni, naming his relatives or friends as vicars of the various lands. He married his son Manfred to the daughter of Amedeo di Savoia and secured the submission of the marquis of Monferrato. On his part, Innocent asked protection from the King of France, Louis IX; but the king was a friend of the Emperor and believed in his desire for peace. A papal army under the command of Ottaviano degli Ubaldini never reached Lombardy, and the Emperor, accompanied by a massive army, held the next diet in Turin.

An unexpected event was to change the situation dramatically. In June 1247 the important Lombard city of Parma expelled the Imperial functionaries and sided with the Guelphs. Enzio was not in the city and could do nothing more than ask for help from his father, who came back to lay siege to the rebels, together with his friend Ezzelino III da Romano, tyrant of Verona. The besieged languished as the Emperor waited for them to surrender from starvation. He had a wooden city, which he called "Vittoria", built around the walls, where he kept his treasure and the harem and menagerie, and from where he could attend his favourite hunting expeditions. On 18 February 1248, during one of these absences, the camp was suddenly assaulted and taken, and in the ensuing Battle of Parma the Imperial side was routed. Frederick lost the Imperial treasure and with it any hope of maintaining the impetus of his struggle against the rebellious communes and against the pope, who began plans for a crusade against Sicily. Frederick soon recovered and rebuilt an army, but this defeat encouraged resistance in many cities that could no longer bear the fiscal burden of his regime: Romagna, Marche and Spoleto were lost.

In February 1249 Frederick fired his advisor and prime minister, the famous jurist and poet Pier delle Vigne on charges of peculation and embezzlement. Some historians suggest that Pier was planning to betray the Emperor, who, according to Matthew of Paris, cried when he discovered the plot. Pier, blinded and in chains, died in Pisa, possibly by suicide. Even more shocking for Frederick was the capture of his son Enzio of Sardinia by the Bolognese at the Battle of Fossalta, in May of the same year. Only twenty - three at the time, he was held in a palace in Bologna, where he remained captive until his death in 1272. Frederick lost another son, Richard of Chieti. The struggle continued: the Empire lost Como and Modena, but regained Ravenna. An army sent to invade the Kingdom of Sicily under the command of Cardinal Pietro Capocci was crushed in the Marche at the Battle of Cingoli in 1250. In the first month of that year the indomitable Ranieri of Viterbo died and the Imperial condottieri again reconquered Romagna, Marche and Spoleto, and Conrad, King of the Romans scored several victories in Germany against William of Holland.

Frederick did not take part in of any of these campaigns. He had been ill and probably felt himself tired. Despite the betrayals and the setbacks he had faced in his last years, Frederick died peacefully, wearing the habit of a Cistercian monk, on 13 December 1250 in Castel Fiorentino (territory of Torremaggiore), in Puglia, after an attack of dysentery. At the time of his death, his preeminent position in Europe was challenged but not lost: his testament left his legitimate son Conrad IV the Imperial and Sicilian crowns. Manfred received the principate of Taranto and the government of the Kingdom, Henry the Kingdom of Arles or that of Jerusalem, while the son of Henry VII was entrusted with the Duchy of Austria and the Marquisate of Styria. Frederick's will stipulated that all the lands he had taken from the Church were to be returned to it, all the prisoners freed, and the taxes reduced, provided this did not damage the Empire's prestige.

However, upon Conrad's death a mere four years later, the Hohenstaufen dynasty fell from power and the Great Interregnum began, lasting until 1273, one year after the last Hohenstaufen, Enzio, had died in his prison. During this time, a legend developed that Frederick was not truly dead but merely sleeping in the Kyffhäuser Mountains and would one day awaken to reestablish his empire. Over time, this legend largely transferred itself to his grandfather, Frederick I, also known as Barbarossa ("Redbeard").

His sarcophagus (made of red porphyry) lies in the cathedral of Palermo beside those of his parents (Henry VI and Constance) as well as his grandfather, the Norman king Roger II of Sicily. A bust of Frederick sits in the Walhalla temple built by Ludwig I of Bavaria.

His contemporaries called Frederick stupor mundi, the "wonder" — or, more precisely, the "astonishment" — "of the world"; the majority of his contemporaries, subscribing to medieval religious orthodoxy, under which the doctrines promulgated by the Church were supposed to be uniform and universal, were, indeed astonished — and sometimes repelled — by the pronounced individuality of the Hohenstaufen emperor, his temperamental stubbornness, and his unorthodox, nearly unquenchable thirst for knowledge.

Frederick II inherited German, Norman, and Sicilian blood, but by training, lifestyle, and temperament he was "most of all Sicilian.". Maehl concludes that, "To the end of his life he remained above all a Sicilian grand signore, and his whole imperial policy aimed at expanding the Sicilian kingdom into Italy rather than the German kingdom southward. Cantor concludes, "Frederick had no intention of giving up Naples and Sicily, which were the real strongholds of its power. He was, in fact, uninterested in Germany.".

Frederick II was a religious skeptic. He is said to have denounced Moses, Jesus, and Muhammad as all being frauds and deceivers of mankind. He delighted in uttering blasphemies and making mocking remarks directed toward Christian sacraments and beliefs. Frederick's religious skepticism was unusual for the era in which he lived, and to his contemporaries, highly shocking and scandalous, and his papal enemies used it against him at every turn.

In Palermo, where the three year old boy was brought after his mother's death, he was said to have grown up like a street youth. He was highly precocious. The only benefit from Innocent III's guardianship was that at fourteen years of age he married a twenty five year old widow named Constance, the daughter of the king of Aragon. Both seem to have been happy with the arrangement, and Constance soon bore a son, Henry.

At his coronation, he may have worn the red silk mantle that had been crafted during the reign of Roger II. It bore an Arabic inscription indicating that the robe dated from the year 528 in the Muslim calendar, and incorporated a generic benediction, wishing its wearer "vast prosperity, great generosity and high splendor, fame and magnificent endowments, and the fulfillment of his wishes and hopes. May his days and nights go in pleasure without end or change". This coronation robe can be found today in the Schatzkammer of the Kunsthistorisches Museum in Vienna.

Rather than exterminate the Saracens of Sicily, he allowed them to settle on the mainland and build mosques. Not least, he enlisted them in his Christian army and even into his personal bodyguards. As Muslim soldiers, they had the advantage of immunity from papal excommunication. For these reasons, as well as his supposed Epicureanism, Frederick II is listed as a representative member of the sixth region of Dante's Inferno, The Heretics who are burned in tombs.

A further example of how much Frederick differed from his contemporaries was the conduct of his Crusade in the Holy Land. Outside Jerusalem, with the power to take it, he parlayed five months with the Ayyubid Sultan of Egypt al-Kamil about the surrender of the city. The Sultan summoned him into Jerusalem and entertained him in the most lavish fashion. When the muezzin, out of consideration for Frederick, failed to make the morning call to prayer, the emperor declared: "I stayed overnight in Jerusalem, in order to overhear the prayer call of the Muslims and their worthy God". The Saracens had a good opinion of him, so it was no surprise that after five months Jerusalem was handed over to him, taking advantage of the war difficulties of al-Kamil. The fact that this was regarded in the Arab as in the Christian world as high treason did not matter to him. When certain members of the Knights Templar wrote al-Kamil a letter and offered to destroy Frederick if he lent them aid, al-Kamil handed the letter over to Frederick. As the Patriarch of Jerusalem refused to crown him king, he set the crown on his own head.

Besides his great tolerance (which, however, did not apply to Christian heretics), Frederick had an unlimited thirst for knowledge and learning. Frederick employed Jews from Sicily, who have immigrated there from the holy land, at his court to translate Greek and Arabic works.

To the dislike of some of his contemporaries, Frederick simply did not believe things that could not be explained by reason. He forbade trials by ordeal in the firm conviction that in a duel the stronger would always win, whether or not he was guilty. Many of his laws continue to influence modern attitudes, such as his prohibition on physicians acting as their own pharmacists. This was a blow to the charlatanism under which physicians diagnosed dubious maladies in order to sell useless, even dangerous "cures".

Frederick II is the author of the first treatise on the subject of falconry, De Arte Venandi cum Avibus ("The Art of Hunting with Birds"). In the words of the historian C.H. Haskins:

It is a scientific book, approaching the subject from Aristotle but based closely on observation and experiment throughout, Divisivus et Inquisitivus, in the words of the preface, it is at the same time a scholastic book, minute and almost mechanical in its divisions and subdivisions. It is also a rigidly practical book, written by a falconer for falconers and condensing a long experience into systematic form for the use of others.

Frederick’s pride in his mastery of the art is illustrated by the story that, when he was ordered to become a subject of the Great Khan (Batu) and receive an office at the Khan’s court, he remarked that he would make a good falconer, for he understood birds very well. He maintained up to fifty falconers at a time in his court, and in his letters he requested Arctic gyrfalcons from Lübeck and even from Greenland. One of the two existing versions was modified by his son Manfred, also a keen falconer.

Frederick loved exotic animals in general: his menagerie, with which he impressed the cold cities of Northern Italy and Europe, included hounds, giraffes, cheetahs, lynxes, leopards, exotic birds and an elephant.

He was also alleged to have carried out a number of experiments on people. These experiments were recorded by the monk Salimbene di Adam (who despised Frederick) in his Chronicles. Amongst the experiments included shutting a prisoner up in a cask to see if the soul could be observed escaping though a hole in the cask when the prisoner died; feeding two prisoners, sending one out to hunt and the other to bed and then have them disemboweled to see which had digested their meal better; imprisoning children without any contact to see if they would develop a natural language.

In the Language deprivation experiment young infants were raised without human interaction in an attempt to determine if there was a natural language that they might demonstrate once their voices matured. It is claimed he was seeking to discover what language would have been imparted unto Adam and Eve by God. In his Chronicles Salimbene wrote that Frederick bade "foster mothers and nurses to suckle and bathe and wash the children, but in no ways to prattle or speak with them; for he would have learnt whether they would speak the Hebrew language (which had been the first), or Greek, or Latin, or Arabic, or per chance the tongue of their parents of whom they had been born. But he laboured in vain, for the children could not live without clappings of the hands, and gestures, and gladness of countenance, and blandishments."

Frederick was also interested in the stars, and his court was host to many astrologers and astronomers, including Michael Scot and Guido Bonatti. He often sent letters to the leading scholars of the time (not only in Europe) asking for solutions to questions of science, mathematics and physics.

In 1224 he founded the University of Naples, the world's oldest state university: now called Università Federico II, it remained the sole atheneum of Southern Italy for centuries.

A Damascene chronicler, Sibt ibn al-Jawzi, left a physical description of Frederick based on the testimony of those who had seen the emperor in person in Jerusalem: "The Emperor was covered with red hair, was bald and myopic. Had he been a slave, he would not have fetched 200 dirhams at market." Frederick's eyes were described variously as blue, or "green like those of a serpent".

His 1241 Edict of Salerno (sometimes called Constitution of Salerno) made the first legally fixed separation of the occupations of physician and apothecary. Physicians were forbidden to double as pharmacists and the prices of various medicinal remedies were fixed. This became a model for regulation of the practice of pharmacy throughout Europe.

He was not able to extend his legal reforms beyond Sicily to the Empire. In 1232, he was forced by the German princes to promulgate the Statutum in favorem principum ("statute in favor of princes"). It was a charter of liberties for the leading German princes at the expense of the lesser nobility and commoners. The princes gained whole power of jurisdiction, and the power to strike their own coins. The emperor lost his right to establish new cities, castles and mints over their territories. The Statutum severely weakened central authority in Germany. From 1232 the vassals of the emperor had a veto over imperial legislative decisions. Every new law established by the emperor had to be approved by the princes.

Historians rate Frederick II as the foremost European monarch of the Middle Ages. This reputation was present even in Frederick's era. Many modern medievalists reject the notion, sponsored by the popes, of Frederick as an anti-Christian as false. They argue that Frederick understood himself as a Christian monarch in the sense of a Byzantine emperor, thus as God's Viceroy on earth. Whatever his personal feelings toward religion, certainly submission to the pope did not enter into the matter. This was in line with the Hohenstaufen Kaiseridee, the ideology claiming the Holy Roman Emperor to be the legitimate successor to the Roman Emperors.

Twentieth century treatments of Frederick vary from the sober (Wolfgang Stürner) to the dramatic (Ernst Kantorowicz). However, all agree on Frederick II's significance as Holy Roman Emperor. In the judgment of British historian Geoffrey Barraclough, Frederick's extensive concessions to German princes — which he made in the hopes of securing his base for his Italian projects — undid the political power of his predecessors and postponed German unity for centuries.