As soon as the Saxons learned that Charlemagne was engaged in Italy, throwing their promises to the winds, they rose again, destroyed a number of Christian churches, and advanced to lay siege to Eresburg, which was occupied by the Franks. Failing to capture the stronghold by assault, they resorted to trickery. By a pretended retreat they induced the Franks to make a sally, then turned upon them, slaughtered them, and demolished the fortress. A few of the garrison saved themselves by flight to Siegburg on the Ruhr,[32] which was attacked by the Saxons without success. Charlemagne, in the meantime, having returned from Italy, suddenly appeared in Saxony and overcame all opposition. He once more pardoned those who implored mercy, restored Eresburg, and built the fortress of Lippestadt. To appease the King, several of the nobles, among them Bruno, son-in-law of Wittekind,[33] accepted Christian baptism and remained as hostages with the King. Charlemagne did not avenge this disloyalty upon his hostages, but continued his efforts to overcome opposition by mild measures which were not altogether satisfactory to his leaders. In his opinion the time had not yet come to undertake forcible conversions, for he was convinced that Christian belief and faith could not be imposed by violence. He was fully resolved to Christianize the Saxons, but he had other methods in view of bringing about that result. He was equally determined that the Saxons should become a political element in the great German nation, but he was cautious about taking any measures that were not absolutely necessary. It was Charlemagne’s custom to call an annual assembly of the leaders of his people upon the Champ de Mai[34] to discuss affairs of state. He decided that year (777) to hold it in Saxony, and selected for its locality the district at the source of the Lippe near Paderborn. He hoped the Saxons would regard this gathering as a peace measure. Their leaders were invited to participate and appeared in a body, with the exception of Wittekind, who bitterly hated the Franks. He had escaped after the defeat; and as Adalgis sought assistance from the Greek Emperor, so he appealed to his brother-in-law Siegfried, King of Denmark, to aid him.