Charles returned to Tuscany and laid siege to the fortress of Poggibonsi, but it did not fall until the end of November. Manfred's staunchest supporters had meanwhile fled to Bavaria to attempt to persuade Conrad IV's 15-year-old son Conradin to assert his hereditary right to the Kingdom of Sicily. After Conradin accepted their proposal, Manfred's former vicar in Sicily, Conrad Capece, returned to the island and stirred up a revolt. At Capece's request Muhammad I al-Mustansir, the Hafsid caliph of Tunis, allowed Manfred's former ally, Frederick of Castile, to invade Sicily from North Africa. Frederick's brother, Henrywho had been elected senator of Romealso offered support to Conradin. Henry had been Charles' friend, but Charles had failed to repay a loan to him.