 Part of the rebellious forces held out , however , and their stronghold was the virtually impregnable Kenilworth Castle . In the summer of 1266 , a siege of the castle was initiated , but the effort proved futile . There were rumours that Montfort 's son Simon was planning an invasion of England from Normandy , and this was the hope that the rebels hung on to . It was in this situation that the papal legate Ottobuono Fieschi exerted his influence , to make the king pursue a more conciliatory policy . In August , the king summoned a parliament at Kenilworth , where the siege was ongoing . He commissioned a number of earls , barons and bishops to draft a treaty of reconciliation . 
