The Castello dell’Imperatore was built on the spot previously occupied by a stronghold of the local Alberti family. In the eleventh century the settlement that had sprung up around this Castrum Prati was merged with the village of Borgo al Cornio, and that is how the city of Prato was born. The Alberti became the counts of Prato, the result of an investiture by the Holy Roman emperor. However, in 1107 they were decisively defeated by one of the Pope’s loyal allies, Matilda of Canossa. Until her death in 1115 this powerful noblewoman served as margravine of Tuscany. Matilda laid siege to Prato, took the city and destroyed the Alberti stronghold. A new stronghold was built towards the end of the twelfth century, around 1185. Two towers of this new building have been incorporated into the present castle (those without battlements). The rest of the castle was built around 1237 by order of the Imperatore after whom the castle takes its name: Frederick II of Hohenstaufen, king of Sicily, king of Germany and Italy, Holy Roman emperor and even king of Jerusalem.
Frederick was born on 26 December 1194 in the small town of Jesi in the Marche. His father was the emperor Henry VI, his mother Constance of Sicily, a posthumous daughter of king Roger II, who had died in 1154. Frederick never really knew his parents, as Henry died in 1197 and Constance the next year. After their deaths the new Pope Innocentius III (1198-1216) took care of the young orphan and became his guardian. Cencio Savelli – the future Pope Honorius III (1216-1227) – was appointed as the boy’s tutor. This boy, a mere four years old, was already king of Sicily, a former superpower that had previously held territories in Northern Africa and had almost brought the Eastern Roman Empire to its knees. The position of Holy Roman emperor eluded Frederick for the moment, as Henry VI was first succeeded by his brother Philip of Swabia and then by Otto of Brunswick. Frederick later successfully took up arms against the latter. When Otto – known to posterity as Otto IV – had been soundly defeated by the French at Bouvines, Frederick became the new king of the Romans. In 1220 he was crowned emperor by his former tutor Pope Honorius III.
Frederick was undoubtedly one of the greatest monarchs of the thirteenth century, as is evidenced by his nickname stupor mundi, which is Latin for “astonishment of the world”. The emperor was a great intellectual, who was very interested in science and was fluent in multiple languages, including Arabic. His proficiency in that language was hardly surprising. Frederick had long been king of Norman Sicily, which was home to a substantial minority of Arabic-speaking Muslims. They were the descendants of the conquerors who had founded an emirate on Sicily in the ninth century. This emirate was only completely defeated by Norman armies in 1091. The Norman conquest did not immediately end the influence of Islam on Sicily. Even in Frederick’s days, more than a century later, large groups of Muslims still inhabited the island. Frederick himself was of course a Catholic monarch, but his interest in Islam was genuine and he greatly admired the works of Islamic scholars. In an era of religious fanaticism and crusades, this gave him a bad reputation, especially with Pope Honorius’ successors. Frederick constantly found himself at odds with these gentlemen, Popes Gregorius IX (1227-1241) and Innocentius IV (1243-1254).
As early as 1217 Frederick had promised Honorius to go on a crusade. The holy city of Jerusalem had been captured by the armies of the First Crusade in 1099, but in 1187 it had been retaken by the Muslims. Frederick made preparations for a campaign to recapture the city, but kept procrastinating his departure for Palestine. When he finally started his crusade in 1227, Honorius had already died. Moreover, Frederick himself was struck by a serious illness. Honorius’ successor Gregorius IX showed little sympathy and excommunicated the emperor for breaking his promise, an act he repeated the next year when a recovered Frederick sailed to Jerusalem. In spite of these setbacks, Frederick continued his crusade and – still excommunicated – concluded a ten-year peace treaty with sultan Al-Kamil of Cairo. The treaty stipulated that Jerusalem, Nazareth and Bethlehem were returned to the Christians. Not a single drop of blood had been shed. Frederick was crowned king of Jerusalem and the city would remain in Christian hands until 1244.
Although the emperor had entered the holiest city in all of Christendom as an excommunicated sinner, Pope Gregorius decided to annul Frederick’s excommunication, only to strike him with a fresh anathema in 1239. Two years previously, Frederick had invaded Italy with his army. Until his death in 1250 the emperor would fight Gregorius, Gregorius’ successor Innocentius IV and the papal party in Italy, the Guelfs. As part of his wars in the peninsula Frederick had the Castello dell’Imperatore built in Prato. The architect was Riccardo da Lentini. As praepositus aedificiorum this architect was responsible for the construction of various other castles, many of which can be found on Sicily. The Castello in Prato is clearly an imperial or Ghibelline castle. Just take a look at the battlements of the towers: these have the shape of a swallow’s tail. The battlements of Guelph castles were rectangular (see Sirmione: Rocca Scaligera). The Castello dell’Imperatore was built as a near-perfect square and has eight towers, including the two that were part of its predecessor (the towers without battlements; see above). The builders made use of pietra alberese quarried near Prato. In some spots we also see some of the famous green marble or marmo verde di Prato.
No matter how splendid and impressive the castle may look on the outside, the Castello dell’Imperatore is actually no more than an empty shell. The courtyard is completely devoid of buildings, which leads to the inevitable conclusion that the castle was never competed after the outer walls had been built. Wooden buildings must have once stood in the courtyard, but these have disappeared long ago. Plans for a stone keep or mastio were no doubt made, but ditched upon the emperor’s death in 1250. A lot had happened in the final five years of Frederick’s life. In 1245 Pope Innocentius IV had instructed the Council of Lyon to depose him. Frederick still had a lot of supporters in Germany, where his son Conrad was king of the Romans, but his adversaries elected Henry Raspe anti-king there. Henry died soon after and was succeeded by count William II of Holland. Supported by, among others, the mighty archbishop of Cologne, William was elected the new anti-king in 1247. William’s cause was greatly aided by Frederick’s death three years later. Conrad followed his father to the grave in 1254, and after the execution of Frederick’s grandson Conradin in 1268, the Hohenstaufen dynasty became extinct.
After Frederick’s death the uncompleted castle was used for multiple purposes. In the 1970s it was thoroughly restored. Nowadays it is mostly used for cultural events. When we visited Prato in August of 2020 it was used as an open-air cinema. The Castello dell’Imperatore can be visited for free and I would highly recommend it. From the walls one has a nice view of, for instance, the churches of San Francesco and Santa Maria delle Carceri.
 Strictly speaking, Gregorius was succeeded by Pope Celestinus IV, but Celestinus’ reign lasted just 17 days, from 25 October until 10 November 1241.