Conradin’s cause seems to prosper, notwithstanding the defection of his uncle Louis and of other companions who have returned to Germany, the threats of Clement IV, and a lack of funds.
Proclaiming him King of Sicily, his partisans, among them Prince Henry of Castile, both in the north and south of Italy have taken up arms; Rome receives his envoy with enthusiasm; and the young king himself receives welcomes at Pavia, Pisa and Siena.
Prince Frederick of Castile, unhappy under the rule of his elder brother Alfonso, he may have participated in the rebellion of his brother Henry in 1255; in any case, he had been exiled from Castile in 1260 and joined Henry as a knight errant in Tunis serving under Sultan Al Mustansir against his enemies.
H later joined the service of King Manfred of Sicily, and in 1266 fought at the Battle of Benevento.
He escaped the defeat there and returned to Tunis.
The anti-Angevin revolt in Sicily in 1267 provides an opportunity for him to cross over again: his Spanish fleet, carrying also a number of knights from Pisa, and Spanish knights soldiering from Tunis, disembarks in the Sicilian city of Sciacca in September 1267, and most of the island rebels against the Angevin rule while Henry (now Senator of Rome) also declares for Conradin in Rome.
Only Palermo and Messina remain loyal to Charles.
The revolt spreads to Calabria and Apulia.